net: skbuff: const-ify casts in skb_queue_* functions

We should const-ify comparisons on skb_queue_* inline helper
functions as their parameters are const as well, so lets not
drop that.

Suggested-by: Brad Spengler <spender@grsecurity.net>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>