vkernel - Fix semi-random SIGILL crashes under heavy network loads

vkernel - Fix semi-random SIGILL crashes under heavy network loads

* SIGIO was being installed with SA_NODEFER, resulting in situations
  under heavy network loads where multiple SIGIOs could stack and cause
  a stack overflow.  This often resulted in the sendsig code in the kernel
  generating a SIGILL to the process after being unable to push a new
  signal context.

* Removing SA_NODEFER appears to solve the problem.

Reported-by: tuxillo