xfs: make the xfs_dir3_icfree_hdr available to xfs_dir2_node_addname_int

Return the xfs_dir3_icfree_hdr used by the helpers called from
xfs_dir2_node_addname_int to the main function to prepare for the
next round of changes where we'll use the ichdr in xfs_dir3_icfree_hdr
to avoid extra operations to find the bests pointers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>