rds: prevent dereference of a NULL device in rds_iw_laddr_check

[ Upstream commit bf39b424 ]

Binding might result in a NULL device which is later dereferenced
without checking.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>