kasan: fix false-positive kasan_report upon thread reuse

In fork1(), if a thread is reused and thread_alloc_stack() is not
called, mark the reused thread's kstack pages clean in the KASAN shadow
buffer.

Sponsored by: Juniper Networks, Inc.
MFC after: 3 days
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44875