General register 0 is clobbered within the inline assembly and
therefore must be listed in the clobber list.

Fixes:  d1e18efa ("s390/lib,uaccess: get rid of register asm")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>