[S390] fix user readable uninitialised kernel memory, take 2.

The previous patch to correct the copy_from_user padding is quite
broken. The execute instruction needs to be done via the register %r4,
not via %r2 and 31 bit doesn't know the instructions lgr and ahji.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>