Avoid entering the slow path of qemu_lockcnt_dec_and_lock if
no bottom half has to be deleted.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20170112180800.21085-11-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>