Holding the rtnl_lock while iterating a devices interface address list
potentially causes deadlocks with the cma_netdev_callback. While this was
implemented to limit the scope of a wildcard listen to addresses of the
current device only, a better solution limits the scope of the socket to
the device. This completely avoiding locking, and also results in
significant code simplification.

Fixes: c421651f ("RDMA/siw: Add missing rtnl_lock around access to ifa")
Link: https://lore.kernel.org/r/20200228173534.26815-1-bmt@zurich.ibm.com
Reported-by: syzbot+55de90ab5f44172b0c90@syzkaller.appspotmail.com
Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>