[bug report] bio: add allocation cache abstraction

[Dan Carpenter]

[ Smatch checking is paused while we raise funding.  #SadFace
  https://lore.kernel.org/all/aTaiGSbWZ9DJaGo7@stanley.mountain/ -dan ]

Hello Jens Axboe,

Commit be4d234d7aeb ("bio: add allocation cache abstraction") from
Mar 8, 2021 (linux-next), leads to the following Smatch static
checker warning:

	block/bio.c:790 bio_cpu_dead()
	error: potential null dereference 'bs'.  (hlist_entry_safe() returns null)

block/bio.c
    785 static int bio_cpu_dead(unsigned int cpu, struct hlist_node *node)
    786 {
    787         struct bio_set *bs;
    788 
    789         bs = hlist_entry_safe(node, struct bio_set, cpuhp_dead);
--> 790         if (bs->cache) {

It doesn't really make sense to use hlist_entry_safe() instead of
hlist_entry() if we're not going to check for NULL.

    791                 struct bio_alloc_cache *cache = per_cpu_ptr(bs->cache, cpu);
    792 
    793                 bio_alloc_cache_prune(cache, -1U);
    794         }
    795         return 0;
    796 }

regards,
dan carpenter