net: caif: Add a missing rcu_read_unlock() in caif_flow_cb

Add a missing rcu_read_unlock in the error path

Fixes: c95567c8 ("caif: added check for potential null return")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>