powerpc/ftrace: Fix stack teardown in ftrace_no_trace

Commit 41a506e ("powerpc/ftrace: Create a dummy stackframe to fix
stack unwind") added use of a new stack frame on ftrace entry to fix
stack unwind. However, the commit missed updating the offset used while
tearing down the ftrace stack when ftrace is disabled. Fix the same.

In addition, the commit missed saving the correct stack pointer in
pt_regs. Update the same.

Fixes: 41a506e ("powerpc/ftrace: Create a dummy stackframe to fix stack unwind")
Cc: [email protected]
Signed-off-by: Naveen N Rao <[email protected]>