skbuff: Extract list pointers to silence compiler warnings

skbuff: Extract list pointers to silence compiler warnings

Under both -Warray-bounds and the object_size sanitizer, the compiler is
upset about accessing prev/next of sk_buff when the object it thinks it
is coming from is sk_buff_head. The warning is a false positive due to
the compiler taking a conservative approach, opting to warn at casting
time rather than access time.

However, in support of enabling -Warray-bounds globally (which has
found many real bugs), arrange things for sk_buff so that the compiler
can unambiguously see that there is no intention to access anything
except prev/next.  Introduce and cast to a separate struct sk_buff_list,
which contains _only_ the first two fields, silencing the warnings:

In file included from ./include/net/net_namespace.h:39,
                 from ./include/linux/netdevice.h:37,
                 from net/core/netpoll.c:17:
net/core/netpoll.c: In function 'refill_skbs':
./include/linux/skbuff.h:2086:9: warning: array subscript 'struct sk_buff[0]' is partly outside array bounds of 'struct sk_buff_head[1]' [-Warray-bounds]
 2086 |         __skb_insert(newsk, next->prev, next, list);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/core/netpoll.c:49:28: note: while referencing 'skb_pool'
   49 | static struct sk_buff_head skb_pool;
      |                            ^~~~~~~~

This change results in no executable instruction differences.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20211207062758.2324338-1-keescook@chromium.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>