xfs: move the zero records logic into xfs_bmap_broot_space_calc

xfs: move the zero records logic into xfs_bmap_broot_space_calc

The bmap btree cannot ever have zero records in an incore btree block.
If the number of records drops to zero, that means we're converting the
fork to extents format and are trying to remove the tree.  This logic
won't hold for the future realtime rmap btree, so move the logic into
the bmbt code.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>