net: remove a stale comment for dl_next

dl_next member in struct request_sock doesn't need to be first.

We expect to insert a "struct common_sock" or a subset of it,
so this claim had to be verified.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>