io_uring: fix poll/netmsg alloc caches

We increase cache->nr_cached when we free into the cache but don't
decrease when we take from it, so in some time we'll get an empty
cache with cache->nr_cached larger than IO_ALLOC_CACHE_MAX, that fails
io_alloc_cache_put() and effectively disables caching.

Fixes: 9b797a3 ("io_uring: add abstraction around apoll cache")
Cc: stable@vger.kernel.org
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/alloc_cache.h

@@ -27,6 +27,7 @@ static inline struct io_cache_entry *io_alloc_cache_get(struct io_alloc_cache *c
 		struct hlist_node *node = cache->list.first;
 
 		hlist_del(node);
+		cache->nr_cached--;
 		return container_of(node, struct io_cache_entry, node);
 	}