KVM: s390: Move common code of mem_op functions into function

KVM: s390: Move common code of mem_op functions into function

The vcpu and vm mem_op ioctl implementations share some functionality.
Move argument checking into a function and call it from both
implementations. This allows code reuse in case of additional future
mem_op operations.

Suggested-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Link: https://lore.kernel.org/r/20230206164602.138068-9-scgl@linux.ibm.com
Message-Id: <20230206164602.138068-9-scgl@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>