net: core: another layer of lists, around PF_MEMALLOC skb handling

[ Upstream commit 78ed8cc25986ac5c21762eeddc1e86e94d422e36 ]

First example of a layer splitting the list (rather than merely taking
 individual packets off it).
Involves new list.h function, list_cut_before(), like list_cut_position()
 but cuts on the other side of the given entry.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[sl: cut out non list.h bits, we only want list_cut_before]
Signed-off-by: Sasha Levin <sashal@kernel.org>