KVM: Don't bother the mmu if cr3 load doesn't change cr3

If the guest requests just a tlb flush, don't take the vm lock and
drop the mmu context pointlessly.

Signed-off-by: Avi Kivity <avi@qumranet.com>

diff --cc drivers/kvm/mmu.c
Simple merge

diff --cc drivers/kvm/x86.c
Simple merge

diff --cc drivers/kvm/x86.h
Simple merge