KVM: s390: fix guarded storage control register handling

store_regs_fmt2() has an ordering problem: first the guarded storage
facility is enabled on the local cpu, then preemption disabled, and
then the STGSC (store guarded storage controls) instruction is
executed.

If the process gets scheduled away between enabling the guarded
storage facility and before preemption is disabled, this might lead to
a special operation exception and therefore kernel crash as soon as
the process is scheduled back and the STGSC instruction is executed.

Fixes: 4e0b1ab72b8a ("KVM: s390: gs support for kvm guests")
Signed-off-by: Heiko Carstens <[email protected]>
Reviewed-by: Christian Borntraeger <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Janosch Frank <[email protected]>
Reviewed-by: Cornelia Huck <[email protected]>
Cc: <[email protected]> # 4.12
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Borntraeger <[email protected]>

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