md/raid5: Convert stripe_head's "dev" to flexible array member

Replace old-style 1-element array of "dev" in struct stripe_head with
modern C99 flexible array. In the future, we can additionally annotate
it with the run-time size, found in the "disks" member.

Cc: Song Liu <song@kernel.org>
Cc: linux-raid@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/lkml/20230522212114.gonna.589-kees@kernel.org/
---
It looks like this memory calculation:

        memory = conf->min_nr_stripes * (sizeof(struct stripe_head) +
                 max_disks * ((sizeof(struct bio) + PAGE_SIZE))) / 1024;

... was already buggy (i.e. it included the single "dev" bytes in the
result). However, I'm not entirely sure if that is the right analysis,
since "dev" is not related to struct bio nor PAGE_SIZE?

drivers/md/raid5.c

@@ -2433,7 +2433,7 @@ static int grow_stripes(struct r5conf *conf, int num)
 
 	conf->active_name = 0;
 	sc = kmem_cache_create(conf->cache_name[conf->active_name],
-			       sizeof(struct stripe_head)+(devs-1)*sizeof(struct r5dev),
+			       struct_size_t(struct stripe_head, dev, devs),
 			       0, 0, NULL);
 	if (!sc)
 		return 1;
@@ -2559,7 +2559,7 @@ static int resize_stripes(struct r5conf *conf, int newsize)
 
 	/* Step 1 */
 	sc = kmem_cache_create(conf->cache_name[1-conf->active_name],
-			       sizeof(struct stripe_head)+(newsize-1)*sizeof(struct r5dev),
+			       struct_size_t(struct stripe_head, dev, newsize),
 			       0, 0, NULL);
 	if (!sc)
 		return -ENOMEM;

drivers/md/raid5.h

@@ -268,7 +268,7 @@ struct stripe_head {
 		unsigned long	flags;
 		u32		log_checksum;
 		unsigned short	write_hint;
-	} dev[1]; /* allocated with extra space depending of RAID geometry */
+	} dev[]; /* allocated depending of RAID geometry ("disks" member) */
 };
 
 /* stripe_head_state - collects and tracks the dynamic state of a stripe_head