s390/page: Add memory clobber to page_set_storage_key()

Add memory clobbers to the page_set_storage_key() inline assemblies. This
allows for data dependencies from other code, which is important to prevent
the compiler from reordering instructions if required.

Note that this doesn't fix a bug in existing code; this is just a
prerequisite for upcoming code changes.

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>