[S390] etr/stp: fix possible deadlock

Precreate stop_machine threads in case the machine supports ETR/STP.
Otherwise we might deadlock if a time sync operation gets scheduled
and the creation of stop_machine threads would cause disk I/O.
This is just the minimal fix.
The real fix would be to only precreate stop_machine threads if
ETR/STP is actually used. But that would be a rather large and
complicated patch.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>