KVM: arm64: Fix detection of shared VMAs on guest fault

When merging the KVM MTE support, the blob that was interposed between
the chair and the keyboard experienced a neuronal accident (also known
as a brain fart), turning a check for VM_SHARED into VM_PFNMAP as it
was reshuffling some of the code.

The blob having now come back to its senses, let's restore the
initial check that the original author got right the first place.

Fixes: ea7fc1bb

 ("KVM: arm64: Introduce MTE VM feature")
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210713114804.594993-1-maz@kernel.org