net: shrink napi_skb_cache_{put,get}() and napi_skb_cache_get_bulk()

Following loop in napi_skb_cache_put() is unrolled by the compiler
even if CONFIG_KASAN is not enabled:

for (i = NAPI_SKB_CACHE_HALF; i < NAPI_SKB_CACHE_SIZE; i++)
kasan_mempool_unpoison_object(nc->skb_cache[i],
kmem_cache_size(net_hotdata.skbuff_cache));

We have 32 times this sequence, for a total of 384 bytes.

48 8b 3d 00 00 00 00 net_hotdata.skbuff_cache,%rdi
e8 00 00 00 00 call kmem_cache_size

This is because kmem_cache_size() is not an inline and not const,
and kasan_unpoison_object_data() is an inline function.

Cache kmem_cache_size() result in a variable, so that
the compiler can remove dead code (and variable) when/if
CONFIG_KASAN is unset.

After this patch, napi_skb_cache_put() is inlined in its callers,
and we avoid one kmem_cache_size() call in napi_skb_cache_get()
and napi_skb_cache_get_bulk().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20251016182911.1132792-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>