netlink: fix netlink_diag_dump() return value

__netlink_diag_dump() returns 1 if the dump is not complete,
zero if no error occurred.

If err variable is zero, this means the dump is complete:
We should not return skb->len in this case, but 0.

This allows NLMSG_DONE to be appended to the skb.
User space does not have to call us again only to get NLMSG_DONE.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>