xfs: move in-memory buftarg handling out of _xfs_buf_ioapply

No I/O to apply for in-memory buffers, so skip the function call
entirely.  Clean up the b_io_error initialization logic to allow
for this.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>