KVM: Make kvm_mmu_slot_remove_write_access() take mmu_lock by itself

Better to place mmu_lock handling and TLB flushing code together since
this is a self-contained function.

Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Gleb Natapov <gleb@redhat.com>