net: add kfree_skb_list()

Extracted from upstream commit bd8a7036c06c "gre: fix a possible skb leak".

This patch adds a kfree_skb_list() helper.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc include/linux/skbuff.h
Simple merge

diff --cc net/core/skbuff.c
Simple merge