Rework the pmap ASID handling to not require the SCHED_LOCK

Rework the pmap ASID handling to not require the SCHED_LOCK

The ASID lookup code already uses its own mutex (pmap_asid_mtx)
and this mutex is enough to ensure that pmap_rollover_asid()
is safe. Now only the generation number check in pmap_setttb()
is done without the lock but the update of the pmap_asid_gen
is now atomic so using READ_ONCE there is enough.

OK kettenis@ mpi@