kernel - Fix issue w/ buffer ortation when doing non-blocking read from bpf

kernel - Fix issue w/ buffer ortation when doing non-blocking read from bpf

* Non-blocking reads from a BPF device not in immediate mode would not
  rotate the buffers even if the was data in the store buffer, but not in
  the hold buffer.  So until the store buffer fills up, the reads would
  return -1 and set errno to EWOULDBLOCK.

Submitted-by: Guy Harris <guy@alum.mit.edu>
Taken-from: FreeBSD