s390/uaccess: Prevent kprobes on cmpxchg_user_key() functions

Code regions within cmpxchg_user_key() functions may be executed with a
non-default access key, which may lead to a protection exception if the
corresponding page has the fetch-protection bit enabled.

There is code in place which initializes the storage keys of such pages
when needed. However there is also the possibility of out-of-line execution
of such code in case a kprobe is set within such a region.

To avoid this problem prevent that any kprobe can be set within the
cmpxchg_user_key() functions.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>