xfs_io: Allow partial writes

xfs_io: Allow partial writes

Partial writes are performed when there is an error midway
while performing the I/O. Perform the write exactly once and
return the number of bytes written so far, until the error.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
[sandeen@sandeen.net: add O to short help]
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>