ipv4: fix NULL checking in devinet_ioctl()

The NULL pointer check `!ifa' should come before its first use.

[ Bug origin : commit fd23c3b3
  (ipv4: Add hash table of interface addresses) in linux-2.6.39 ]

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>