KVM: arm/arm64: VGIC/ITS: protect kvm_read_guest() calls with SRCU lock

KVM: arm/arm64: VGIC/ITS: protect kvm_read_guest() calls with SRCU lock

kvm_read_guest() will eventually look up in kvm_memslots(), which requires
either to hold the kvm->slots_lock or to be inside a kvm->srcu critical
section.
In contrast to x86 and s390 we don't take the SRCU lock on every guest
exit, so we have to do it individually for each kvm_read_guest() call.

Provide a wrapper which does that and use that everywhere.

Note that ending the SRCU critical section before returning from the
kvm_read_guest() wrapper is safe, because the data has been *copied*, so
we don't need to rely on valid references to the memslot anymore.

Cc: Stable <[email protected]> # 4.8+
Reported-by: Jan Glauber <[email protected]>
Signed-off-by: Andre Przywara <[email protected]>
Acked-by: Christoffer Dall <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>