kasan: wrap kstack cleanup into a helper

[khng]

Summary

MFC with: 68a3a7fc9483
MFC after: 3 days
Sponsored by: Juniper Networks, Inc.

[khng]

.

[markj]

sys/kern/subr_asan.c

434

I'd prefer a more generic name, like kasan_kstack_init().

Or, follow the pattern of KMSAN and have kasan_thread_alloc() which only zeros the shadow of the stack (if td->td_kstack != 0).

[khng]

Adapt markj@'s suggestions.

[markj]

sys/kern/subr_asan.c

434

Sorry, by "follow the pattern of KMSAN" I meant that kmsan_thread_alloc() and kasan_thread_alloc() calls should be in the same place. Otherwise it's rather confusing.

We can actually remove the shadow space marking in vm_thread_new() as well. I posted a diff to this effect: https://reviews.freebsd.org/D44891