kernel - Fix mbuf header exhaustion w/ degenerate unix domain socket case

kernel - Fix mbuf header exhaustion w/ degenerate unix domain socket case

* Increase the number of mbuf headers to accomodate a degenerate
  fd-passing case with chains of unix domain sockets, one fd per
  socket.

* This does not address other mbuf exhaustion issues which can crop
  up in a user ddos attack due to the default number of fds a user
  is allowed to own.

Reported-by: Venkatesh Srinivas <me@endeavour.zapto.org>