KVM: arm/arm64: Re-check VMA on detecting a poisoned page

When we check for a poisoned page, we use the VMA to tell userspace
about the looming disaster. But we pass a pointer to this VMA
after having released the mmap_sem, which isn't a good idea.

Instead, stash the shift value that goes with this pfn while
we are holding the mmap_sem.

Reported-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Link: https://lore.kernel.org/r/20191211165651.7889-3-maz@kernel.org
Link: https://lore.kernel.org/r/20191217123809.197392-1-james.morse@arm.com