MFC r213736: generic_stop_cpus: prevent parallel execution

Addendum to the original commit message:
This is mostly useful to protect kdb_trap() from concurrent entry as it
doesn't have any protection like e.g. panic(9) has. Even if kdb_trap()
had that protection there would still be a need for protection
between concurrect panic() and kdb_trap() entries. That protection
could be implemented externally, but doing it in generic_stop_cpus()
seems to be OK as well (stopping CPUs on panic is in the works).