s390/kmsan: Fix merge conflict with get_lowcore() introduction

Resolve the conflict between commit 2a48c8c9 ("s390/kmsan:
implement the architecture-specific functions") and commit 39976f12
("s390: Remove S390_lowcore").

Fixes: 2a48c8c9

 ("s390/kmsan: implement the architecture-specific functions")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Link: https://lore.kernel.org/r/20240723124441.120044-2-iii@linux.ibm.com
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>