ksmbd: fix refcount leak causing resource not released

When ksmbd_conn_releasing(opinfo->conn) returns true,the refcount was not
decremented properly, causing a refcount leak that prevents the count from
reaching zero and the memory from being released.

Cc: stable@vger.kernel.org
Signed-off-by: Ziyan Xu <ziyan@securitygossip.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>