KVM: s390: ucontrol: Fix sca_clear_ext_call()

KVM: s390: ucontrol: Fix sca_clear_ext_call()

When cleaning up a UCONTROL VM, sca_clear_ext_call() will touch memory
outside of the allocated ESCA block, and UCONTROL VMs don't even use
ESCA.

Fix by not touching ESCA for UCONTROL VMs, and fence the
KVM_S390_INTERRUPT ioctl altogether. Add extra checks in
sca_ext_call_pending() and sca_inject_ext_call() to make sure UCONTROL
VMs won't touch ESCA.

Fencing does not cause regressions with userspace, since UCONTROL VMs
never used KVM_S390_INTERRUPT ioctls.

Fixes: 7d43bafcff17 ("KVM: s390: Make provisions for ESCA utilization")
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-ID: <20260803124040.126471-6-imbrenda@linux.ibm.com>