KVM: arm64: Use lm_alias in nVHE-only VA conversion

init_hyp_physvirt_offset() computes PA from a kernel VA. Conversion to
kernel linear-map is required first but the code used kvm_ksym_ref() for
this purpose. Under VHE that is a NOP and resulted in a runtime warning.
Replace kvm_ksym_ref with lm_alias.

Reported-by: Qian Cai <qcai@redhat.com>
Signed-off-by: David Brazdil <dbrazdil@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201208142452.87237-3-dbrazdil@google.com