Machines can stall out because mbufs are low, however sometimes we won't
see "requests denied", instead we see user land processes or kernel threads
blocking waiting for mbufs because they set M_WAIT. These consumers do not
see errors, only stalling.
Unfortunately until now, netstat did not export this information
so you could have experienced an mbuf shortage and have no way of
seeing it unless you happen to run netstat at the exact time of the
shortage and see "in use" = "max".
By exporting the number of times processes are blocked, we can
effectively see how often non-interrupt context threads are effectively
"denied".
MFC after: 2 weeks
504a74fa90