s390/uaccess: Merge cmpxchg_user_key() inline assemblies

The inline assemblies for __cmpxchg_user_key1() and __cmpxchg_user_key2()
are identical. Get rid of the duplication and provide a common helper
function.

Suggested-by: Mete Durlu <meted@linux.ibm.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>