s390/kprobes: add exrl to list of prohibited opcodes

"execute relative long" may have all sorts of side effects dependend on
the instructions it executes.
Therefore prohibit setting a kprobe on exrl just like we do for the
regular execute instruction.

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