s390/cpcmd: fix inline assembly register clobbering

s390/cpcmd: fix inline assembly register clobbering

Register variables initialized using arithmetic. That leads to
kasan instrumentaton code corrupting the registers contents.
Follow GCC guidlines and use temporary variables for assigning
init values to register variables.

Fixes: 51c990313d04 ("[S390] Inline assembly cleanup.")
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Local-Register-Variables.html
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>