KVM: MMU: Make kvm_mmu_slot_remove_write_access() rmap based

This makes it possible to release mmu_lock and reschedule conditionally
in a later patch.  Although this may increase the time needed to protect
the whole slot when we start dirty logging, the kernel should not allow
the userspace to trigger something that will hold a spinlock for such a
long time as tens of milliseconds: actually there is no limit since it
is roughly proportional to the number of guest pages.

Another point to note is that this patch removes the only user of
slot_bitmap which will cause some problems when we increase the number
of slots further.

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>