s390/extable: move extable related functions to mm/extable.c

Just like arm64, riscv, and x86 move extable related functions to
mm/extable.c. This is currently only one function, but this will
change with subsequent changes.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>