[9fans] 9vx fork problem

[Anthony Martin <ality@pbrane.org>]

On 9vx, iostats suicides:

  term% iostats echo
  iostats 128: suicide: sys: trap: page fault pc=0x0000b21c

Apparently, after a fork, a child retains it's parent's
pid in _tos->pid. It isn't updated until the next syscall
finishes with kexit.

This is a problem if the child's first function call is
malloc. When malloc locks it's pool, it stores the pid
from _tos in a private area. After the kernel returns from
the brk, _tos->pid has been corrected by the call to kexit.
Punlock then fails because the pids don't match.

I'm not sure of the best way to go about fixing this.

     Anthony