net: diag: Add support to filter on device index

Add support to inet_diag facility to filter sockets based on device
index. If an interface index is in the filter only sockets bound
to that index (sk_bound_dev_if) are returned.

[backport of net-next 637c841dd7a5f9bd97b75cbe90b526fa1a52e530]

Change-Id: Ib430cfb44f1b3b1a771a561247ee9140737e52fd
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>