Swap in WKILLED processes.

Swapped-out process that is WKILLED must be swapped in as soon as
possible. The reason is that such process can be killed by OOM and
its pages can be only freed if the process exits. To exit, the kernel
stack of the process must be mapped.

When allocating pages for the stack of the WKILLED process on swap in,
use VM_ALLOC_SYSTEM requests to increase the chance of the allocation
to succeed.

Add counter of the swapped out processes to avoid unneeded iteration
over the allprocs list when there is no work to do, reducing the
allproc_lock ownership.

Reviewed by: alc, markj (previous version)
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D16489