RDMA/siw: Add missing rtnl_lock around access to ifa

ifa is protected by rcu or rtnl, add the missing locking. In this case we
have to use rtnl since siw_listen_address() is sleeping.

Fixes: 6c52fdc2

 ("rdma/siw: connection management")
Reviewed-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>