KVM: arm/arm64: Fix check for hugepage size when allocating at Stage 2

Commit 45ee9d5e ("KVM: arm/arm64: Check pagesize when allocating a
hugepage at Stage 2") lost the check for PMD_SIZE during the backport
to 4.9.

Fix this by correcting the condition to detect hugepages during stage
2 allocation.

Fixes: 45ee9d5e ("KVM: arm/arm64: Check pagesize when allocating a hugepage at Stage 2")
Reported-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>