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.)

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.

Remove an unnecessary field from struct blist.  (The comment describing
what this field represented was also inaccurate.)

In r178792, blist_create() grew a malloc flag, allowing M_NOWAIT to be
specified.  However, blist_create() was not modified to handle the
possibility that a malloc() call failed.  Address this omission.

Increase the width of the local variable "radix" to 64 bits.  This
matches the width of the corresponding field in struct blist.