KVM: Fix the bitmap range to copy during clear dirty

KVM: Fix the bitmap range to copy during clear dirty

kvm_dirty_bitmap_bytes() will return the size of the dirty bitmap of
the memslot rather than the size of bitmap passed over from the ioctl.
Here for KVM_CLEAR_DIRTY_LOG we should only copy exactly the size of
bitmap that covers kvm_clear_dirty_log.num_pages.

Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: stable@vger.kernel.org
Fixes: bfe9d5df8ebe278672c430566c38d8e1f1d9d554
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>