xfs: remove xfs_attr_shortform_lookup

xfs_attr_shortform_lookup is only used by xfs_attr_shortform_addname,
which is much better served by calling xfs_attr_sf_findname.  Switch
it over and remove xfs_attr_shortform_lookup.

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>