Do not use potentially stale thread in kthread_add()

When an existing process is provided, the thread selected to use
to initialize the new thread could have exited and be reaped.
Acquire the proc lock earlier to ensure the thread remains valid.

Reviewed by:	jhb, julian (previous version)
MFC after:	3 days
- Use getnameinfo(3) instead of gethostbyaddr(3) or inet_ntop(3).

- Fill sin6_scope_id from in6p.sin6_addr.s6_addr[2].  struct inpcb has
  struct in6_addr for the endpoint addresses, so sin6_scope_id must be filled.