RDMA/core: Check error status of rdma_find_ndev_for_src_ip_rcu

rdma_find_ndev_for_src_ip_rcu() returns either valid netdev pointer or
ERR_PTR().  Instead of checking for NULL, check for error.

Fixes: caf1e3ae ("RDMA/core Introduce and use rdma_find_ndev_for_src_ip_rcu")
Reported-by:  <syzbot+20c32fa6ff84a2d28c36@syzkaller.appspotmail.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>