Incorrect error handling in io_epoll_context

[igor-anferov]

The code in io_epoll_context implies that readv and writev syscalls error codes are returned as different negative values. Instead, they always return -1, and actual error code is returned through errno variable. The only reason it currently works (even though it always adds fds to epoll even if there's an unrecoverable error, not just EAGAIN, never calling set_error in start_io, and calling set_error with EPERM in case of ANY error, hiding it) is because it compares return code with -EPERM, which turns out to be -1 on Linux, and adds such descriptors to epoll. I wonder where the idea of this check comes from, cause adding such descriptor to epoll wouldn't fix real EPERM error...