i40iw: Do an RCU lookup in i40iw_add_ipv4_addr

The in_dev_for_each_ifa_rtnl() iterator in i40iw_add_ipv4_addr requires
that the rtnl lock be held. But the rtnl_trylock/unlock scheme in this
function does not guarantee it.

Replace the rtnl locking with an RCU lookup using
in_dev_for_each_ifa_rcu()

Fixes: 8e06af71

 ("i40iw: add main, hdr, status")
Link: https://lore.kernel.org/r/20200204223840.2151-1-shiraz.saleem@intel.com
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>