KVM: arm64: Take a pointer to walker data in kvm_dereference_pteref()

Rather than passing through the state of the KVM_PGTABLE_WALK_SHARED
flag, just take a pointer to the whole walker structure instead. Move
around struct kvm_pgtable and the RCU indirection such that the
associated ifdeffery remains in one place while ensuring the walker +
flags definitions precede their use.

No functional change intended.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221118182222.3932898-2-oliver.upton@linux.dev