s390/uaccess: limit number of retries for cmpxchg_user_key()

s390/uaccess: limit number of retries for cmpxchg_user_key()

cmpxchg_user_key() for byte and short values is implemented via a one word
cmpxchg loop. Give up trying to perform the cmpxchg if it fails too often
because of contention on the cache line. This ensures that the thread
cannot become stuck in the kernel.

Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Link: https://lore.kernel.org/r/20221117100745.3253896-1-scgl@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>