s390/kprobes: add sanity check

Check whether the specified address points to the start of an
instruction to prevent users from setting a kprobe in the mid of
an instruction which would crash the kernel.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>