Ignore the P_SYSTEM process flag, and do not request
VM_MAP_WIRE_SYSTEM mode when wiring the newly grown stack.

System maps do not create auto-grown stack. Any stack we handled,
even for P_SYSTEM, must be for user address space. P_SYSTEM processes
with mapped user space is either init(8) or an aio worker attached to
other user process with aio buffer pointing into stack area. In either
case, VM_MAP_WIRE_USER mode should be used.

Noted and reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week