Add support for reinjecting the re-assembled frames back into the
correct CPU after the RSS hash has been re-calculated.

* add an m2cpuid handler for IP netisr frames.
* use netisr_dispatch / netisr_queue in ip_reass() rather than
  returning the frame for further processing.
* add a sysctl to flip between netisr_dispatch and netisr_queue.

netisr_queue() to another netisr context (on another CPU) gives good
performance, but when netisr threads are created 1:1 for CPUs _and_
they're pinned, _and_ nh_m2cpu calls the rss method to map it to
the right local CPU ID, things get very crappy. The netisr context
on the CPU combined with the NIC receive context on the CPU starve
userland on that CPU from making progress, causing queue drops
of quite significant levels.

Also, and more importantly:

* This is much, much slower than doing a direct dispatch of the
  reassembled frame (ie, not going via netisr) - but it's not
  correct.  I'm not yet sure why it's much slower than just
  returning the mbuf.  Le sigh.