fork() may allocate a new thread in one of two ways: from UMA, or cached
in a freed proc that was just allocated from UMA. In either case, KASAN
and KMSAN need to initialize some state; in particular they need to
initialize the shadow mapping of the new thread's stack.

This is done differently between KASAN and KMSAN, which is confusing.
This patch improves things a bit:

• Add a new thread_recycle() function, which moves all kernel stack handling out of kern_fork.c, since it doesn't really belong there.
• Then, thread_alloc_stack() has only one local caller, so just inline it.
• Avoid redundant shadow stack initialization: thread_alloc() initializes the KMSAN shadow stack (via kmsan_thread_alloc()) even through vm_thread_new() already did that.
• Add kasan_thread_alloc(), for consistency with kmsan_thread_alloc().

No functional change intended.