xfs: Add helper function xfs_attr_node_shrink

This patch adds a new helper function xfs_attr_node_shrink used to
shrink an attr name into an inode if it is small enough.  This helps to
modularize the greater calling function xfs_attr_node_removename.

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