net: move skbuff_cache(s) to net_hotdata

skbuff_cache, skbuff_fclone_cache and skb_small_head_cache
are used in rx/tx fast paths.

Move them to net_hotdata for better cache locality.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240306160031.874438-11-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>