kernel - Fix atomic op comparison

kernel - Fix atomic op comparison

* The sequence was testing a signed integer and then testing the same
  variable using atomic_fetchadd_int(&var, 0).  Unfortunately, the
  atomic-op returns an unsigned value so the result is that when the
  buffer count was exhausted, the program would hard-loop without
  calling tsleep.

* Fixed by casting the atomic op.

* Should fix the hardlock issue once and for all.