blist_fill()'s return type is too narrow. blist_fill() accepts a 64-bit
quantity as the size of the range to fill, but returns a 32-bit quantity
as the number of blocks that were allocated to fill that range. This
revision corrects that mismatch. Currently, swaponsomething() limits
the size of a swap area to prevent arithmetic arithmetic overflow in
other parts of the blist allocator. That limit has also prevented this
type mismatch from causing problems.

Reviewed by: kib, markj
MFC after: 6 weeks
Differential Revision: https://reviews.freebsd.org/D11096