KVM: MIPS: rework flush_shadow_* callbacks into one that prepares the flush

Both trap-and-emulate and VZ have a single implementation that covers
both .flush_shadow_all and .flush_shadow_memslot, and both of them end
with a call to kvm_flush_remote_tlbs.

Unify the callbacks into one and extract the call to kvm_flush_remote_tlbs.
The next patches will pull it further out of the the architecture-specific
MMU notifier functions kvm_unmap_hva_range and kvm_set_spte_hva.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>