dm clone: Make __hash_find static

drivers/md/dm-clone-target.c:594:34: warning:
 symbol '__hash_find' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>