xfs: Add helpers xfs_attr_is_shortform and xfs_attr_set_shortform

In this patch, we hoist code from xfs_attr_set_args into two new helpers
xfs_attr_is_shortform and xfs_attr_set_shortform.  These two will help
to simplify xfs_attr_set_args when we get into delayed attrs later.

Signed-off-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Chandan Rajendra <chandanrlinux@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Acked-by: Dave Chinner <dchinner@redhat.com>