xfs: devirtualize ->db_to_fdb and ->db_to_fdindex

Now that the max bests value is in struct xfs_da_geometry both instances
of ->db_to_fdb and ->db_to_fdindex are identical.  Replace them with
local xfs_dir2_db_to_fdb and xfs_dir2_db_to_fdindex functions in
xfs_dir2_node.c.

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>