KVM: x86/mmu: Consolidate open coded variants of memslot TLB flushes
Replace open coded instances of kvm_arch_flush_remote_tlbs_memslot()'s
functionality with calls to the aforementioned function. Update the
comment in kvm_arch_flush_remote_tlbs_memslot() to elaborate on how it
is used and why it asserts that slots_lock is held.
No functional change intended.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>