kernel - Fix a few edge cases in subr_blist.c

kernel - Fix a few edge cases in subr_blist.c

* In the all-allocated special case set bm_bighint to 0 instead of to
  count.  This fixes an edge case where failed allocations wind up
  traversing too much of the radix tree.

* In the all-free special case be sure to set bm_bighint to the radix,
  higher layers check bm_bighint when recursing.  (This bug could not occur
  unless you had 4 swap devices configured).

* Improve code documentation and minor cleanups.