Interrupt routine usage not shown by top in 8.0

[Barney Cordoba]

I'm fireing 400Kpps at a udp blackhole port. I'm getting 6000 interrupts
per second on em3:

testbox# vmstat -i; sleep 1; vmstat -i
interrupt                          total       rate
irq1: atkbd0                           1          0
irq6: fdc0                             1          0
irq17: uhci1+                       2226          9
irq18: uhci2 ehci+                     9          0
cpu0: timer                       470507       1993
irq256: em0                          665          2
irq259: em3                      1027684       4354
cpu1: timer                       470272       1992
cpu3: timer                       470273       1992
cpu2: timer                       470273       1992
Total                            2911911      12338

interrupt                          total       rate
irq1: atkbd0                           1          0
irq6: fdc0                             1          0
irq17: uhci1+                       2226          9
irq18: uhci2 ehci+                     9          0
cpu0: timer                       472513       1993
irq256: em0                          668          2
irq259: em3                      1033703       4361
cpu1: timer                       472278       1992
cpu3: timer                       472279       1992
cpu2: timer                       472279       1992
Total                            2925957      12345


top -SH shows:

  PID  STATE  C   TIME    CPU COMMAND
   10  CPU3   3   7:32 100.00% idle
   10  CPU2   2   7:32 100.00% idle
   10  RUN    0   7:31 100.00% idle
   10  CPU1   1   7:31 100.00% idle

This implies that CPU usage is substantially under-reported in general
by the system. Note that I've modified em_irq_fast() to call 
em_handle_rxtx() directly rather than scheduling a task to illustrate
the problem

Barney