xfs: use xfs_attr_sf_findname in xfs_attr_shortform_getvalue

xfs_attr_shortform_getvalue duplicates the logic in xfs_attr_sf_findname.
Use the helper instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>