ipv4: Get the address of interface correctly.

When using ioctl to get address of interface, we can't
get it anymore. For example, the command is show as below.

	# ifconfig eth0

In the patch ("03aef17b"), the devinet_ioctl does not
return a suitable value, even though we can find it in
the kernel. Then fix it now.

Fixes: 03aef17b ("devinet_ioctl(): take copyin/copyout to caller")
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>