net: kill an RCU warning in inet_fill_link_af()

commits 9f0f7272 (ipv4: AF_INET link address family) and cf7afbfe
(rtnl: make link af-specific updates atomic) used incorrect
__in_dev_get_rcu() in RTNL protected contexts, triggering PROVE_RCU
warnings.

Switch to __in_dev_get_rtnl(), wich is more appropriate, since we hold
RTNL.

Based on a report and initial patch from Amerigo Wang.

Reported-by: Amerigo Wang <amwang@redhat.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Thomas Graf <tgraf@infradead.org>
Reviewed-by: WANG Cong <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>