decnet: add RTNL lock when reading address list

Add missing locking in the case of auto binding to the
default device. The address list might change while this code is looking
at the list.

Compile tested only, I am not a decnet user.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>