When allocating swap blocks, if the available number of free blocks in a
subtree is already zero, then setting the "largest contiguous free block"
hint for that subtree to anything other than zero makes no sense. To be
clear, assigning a value to the hint that is too large is not a correctness
problem, only a pessimization.

Dragonfly BSD has applied the same change to blst_meta_alloc() but not
blst_meta_fill().

MFC after: 6 weeks