bio: shrink max number of pcpu cached bios

The downside of the bio pcpu cache is that bios of a cpu will be never
freed unless there is new I/O issued from that cpu. We currently keep
max 512 bios, which feels too much, half it.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/bc198e8efb27d8c740d80c8ce477432729075096.1667384020.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -27,7 +27,7 @@
 
 #define ALLOC_CACHE_THRESHOLD	16
 #define ALLOC_CACHE_SLACK	64
-#define ALLOC_CACHE_MAX		512
+#define ALLOC_CACHE_MAX		256
 
 struct bio_alloc_cache {
 	struct bio		*free_list;