net: dropreason: add SKB_CONSUMED reason

This will allow to simply use in the future:

	kfree_skb_reason(skb, reason);

Instead of repeating sequences like:

	if (dropped)
	    kfree_skb_reason(skb, reason);
	else
	    consume_skb(skb);

For instance, following patch in the series is adding
@reason to skb_release_data() and skb_release_all(),
so that we can propagate a meaningful @reason whenever
consume_skb()/kfree_skb() have to take care of a potential frag_list.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>