fork: Suspend other threads if both RFPROC and RFMEM are not set
Otherwise, a multithreaded parent process may trigger races in
vm_forkproc() if one thread calls rfork() with RFMEM set and another
calls rfork() without RFMEM.
Also simplify vm_forkproc() a bit, vmspace_unshare() already checks to
see if the address space is shared.
Reported by: email@example.com
Reported by: firstname.lastname@example.org
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30220