xfs: add a bests pointer to struct xfs_dir3_icfree_hdr

All but two callers of the ->free_bests_p dir operation already have a
struct xfs_dir3_icfree_hdr from a previous call to
xfs_dir2_free_hdr_from_disk at hand.  Add a pointer to the bests to
struct xfs_dir3_icfree_hdr to clean up this pattern.  To optimize this
pattern, pass the struct xfs_dir3_icfree_hdr to xfs_dir2_free_log_bests
instead of recalculating the pointer there.

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>