s390/zcrypt: fixed ap poll timer behavior

The ap poll timer restart condition was wrong. Hence the poll timer
was not restarted reliable when setting a new time interval via the
poll_timeout sysfs attribute.
Added missing timer locking.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Tuchscherer <ingo.tuchscherer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>