[S390] kprobes: single step cleanup

The saved interrupt mask and the saved control registers are only
relevant while single stepping is set up. A secondary kprobe while
kprobe single stepping is active may not occur. That makes is safe
to remove the save and restore of kprobe_saved_imask / kprobe_save_ctl
from save_previous_kprobe and restore_previous_kprobe.
Move all single step related code to two functions, enable_singlestep
and disable_singlestep.

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