rds: prevent dereference of a NULL device in rds_iw_laddr_check

[ Upstream commit bf39b4247b8799935ea91d90db250ab608a58e50 ]

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

Change-Id: I2982164d873445c6bc2e5283a05c5b57deb5d11f
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>