arm/arm64: KVM: Take mmap_sem in stage2_unmap_vm

We don't hold the mmap_sem while searching for the VMAs when
we try to unmap each memslot for a VM. Fix this properly to
avoid unexpected results.

Fixes: commit 957db105 ("arm/arm64: KVM: Introduce stage2_unmap_vm")
Cc: stable@vger.kernel.org # v3.19+
Reviewed-by: Christoffer Dall <cdall@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>