zfs: Fix GPF in zfs_rmnode()

If a crashed OSv guest is restarted, ZFS mount causes a GPF in early
startup:

  VFS: mounting zfs at /usr
  zfs: mounting osv/usr from device /dev/vblk1
  Aborted

GDB backtrace points finger at zfs_rmnode():

  #0  processor::halt_no_interrupts () at ../../arch/x64/processor.hh:212
  #1  0x00000000003e7f2a in osv::halt () at ../../core/power.cc:20
  #2  0x000000000021cdd4 in abort (msg=0x636df0 "Aborted\n") at ../../runtime.cc:95
  #3  0x000000000021cda2 in abort () at ../../runtime.cc:86
  #4  0x000000000044c149 in osv::generate_signal (siginfo=..., ef=0xffffc0003ffe7008) at ../../libc/signal.cc:44
  #5  0x000000000044c220 in osv::handle_segmentation_fault (addr=72, ef=0xffffc0003ffe7008) at ../../libc/signal.cc:55
  #6  0x0000000000366df3 in page_fault (ef=0xffffc0003ffe7008) at ../../core/mmu.cc:876
  #7  <signal handler called>
  #8  0x0000000000345eaa in zfs_rmnode (zp=0xffffc0003d1de400)
      at ../../bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_dir.c:611
  #9  0x000000000035650c in zfs_zinactive (zp=0xffffc0003d1de400)
      at ../../bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_znode.c:1355
  #10 0x0000000000345be1 in zfs_unlinked_drain (zfsvfs=0xffffc0003ddfe000)
      at ../../bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_dir.c:523
  #11 0x000000000034f45c in zfsvfs_setup (zfsvfs=0xffffc0003ddfe000, mounting=true)
      at ../../bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vfsops.c:881
  #12 0x000000000034f7a4 in zfs_domount (vfsp=0xffffc0003de02000, osname=0x6b14cb "osv/usr")
      at ../../bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vfsops.c:1016
  #13 0x000000000034f98c in zfs_mount (mp=0xffffc0003de02000, dev=0x6b14d7 "/dev/vblk1", flags=0, data=0x6b14cb)
      at ../../bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vfsops.c:1415
  #14 0x0000000000406852 in sys_mount (dev=0x6b14d7 "/dev/vblk1", dir=0x6b14a3 "/usr", fsname=0x6b14d3 "zfs", flags=0, data=0x6b14cb)
      at ../../fs/vfs/vfs_mount.c:171
  #15 0x00000000003eff97 in mount_usr () at ../../fs/vfs/main.cc:1415
  #16 0x0000000000203a89 in do_main_thread (_args=0xffffc0003fe9ced0) at ../../loader.cc:215
  #17 0x0000000000448575 in pthread_private::pthread::pthread(void* (*)(void*), void*, sigset_t, pthread_private::thread_attr const*)::{lambda()#1}::operator()() const () at ../../libc/pthread.cc:59
  #18 0x00000000004499d3 in std::_Function_handler<void(), pthread_private::pthread::pthread(void* (*)(void*), void*, sigset_t, const pthread_private::thread_attr*)::__lambda0>::_M_invoke(const std::_Any_data &) (__functor=...)
      at ../../external/gcc.bin/usr/include/c++/4.8.1/functional:2071
  #19 0x000000000037e602 in std::function<void ()>::operator()() const (this=0xffffc0003e170038)
      at ../../external/gcc.bin/usr/include/c++/4.8.1/functional:2468
  #20 0x00000000003bae3e in sched::thread::main (this=0xffffc0003e170010) at ../../core/sched.cc:581
  #21 0x00000000003b8c92 in sched::thread_main_c (t=0xffffc0003e170010) at ../../arch/x64/arch-switch.hh:133
  #22 0x0000000000399c8e in thread_main () at ../../arch/x64/entry.S:101

The problem is that ZFS tries to check if the znode is an attribute
directory and trips over zp->z_vnode being NULL.  However, as explained
in commit b7ee91e ("zfs: port vop_lookup"), we don't even support
extended attributes so drop the check completely for OSv.

bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_dir.c

@@ -525,6 +525,7 @@ zfs_unlinked_drain(zfsvfs_t *zfsvfs)
 	zap_cursor_fini(&zc);
 }
 
+#ifdef NOTYET
 /*
  * Delete the entire contents of a directory.  Return a count
  * of the number of entries that could not be deleted. If we encounter
@@ -591,6 +592,7 @@ zfs_purgedir(znode_t *dzp)
 		skipped += 1;
 	return (skipped);
 }
+#endif
 
 void
 zfs_rmnode(znode_t *zp)
@@ -605,6 +607,7 @@ zfs_rmnode(znode_t *zp)
 
 	ASSERT(zp->z_links == 0);
 
+#ifndef __OSV__
 	/*
 	 * If this is an attribute directory, purge its contents.
 	 */
@@ -620,6 +623,7 @@ zfs_rmnode(znode_t *zp)
 			return;
 		}
 	}
+#endif
 
 	/*
 	 * Free up all the data in the file.