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

mainline inclusion
from mainline-v5.13-rc1
commit 5194552f
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I7S3VQ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5194552fb1ffb4248c8db3f7286aa4ac7ae3163b



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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>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>