s390/uprobes: fix user space PER events

If uprobes are single stepped for example with gdb, the behavior should
now be correct. Before this patch, when gdb was single stepping a uprobe,
the result was a SIGILL.
When PER is active for any storage alteration and a uprobe is hit, a storage
alteration event is indicated. These over indications are filterd out by gdb,
if no change has happened within the observed area.

Signed-off-by: Jan Willeke <willeke@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>