RDMA/siw: Fix IPv6 addr_list locking

Walking the address list of an inet6_dev requires
appropriate locking. Since the called function
siw_listen_address() may sleep, we have to use
rtnl_lock() instead of read_lock_bh().

Also introduces sanity checks if we got a device
from in_dev_get() or in6_dev_get().

Reported-by: Bart Van Assche <bvanassche@acm.org>
Fixes: 6c52fdc2

 ("rdma/siw: connection management")
Signed-off-by: Bernard Metzler <bmt@zurich.ibm.com>
Link: https://lore.kernel.org/r/20190828130355.22830-1-bmt@zurich.ibm.com
Signed-off-by: Doug Ledford <dledford@redhat.com>