KVM: s390: Fix ordering when adding to SCA

KVM: s390: Fix ordering when adding to SCA

When adding a new vCPU to the SCA area, the validity bit in the MCN was
set before the pointer to the state description, potentially allowing
for a race.

Fix by setting the pointer before setting the bit.

Fixes: 14542a0a54c5 ("KVM: S390: Remove sca_lock")
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-ID: <20260803124040.126471-13-imbrenda@linux.ibm.com>