dm: replace zero-length array with flexible-array

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
int stuff;
struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-crypt.c

···
 	struct mutex bio_alloc_lock;
 
 	u8 *authenc_key; /* space for keys in authenc() format (if used) */
-	u8 key[0];
+	u8 key[];
 };
 
 #define MIN_IOS		64

drivers/md/dm-integrity.c

···
 		} s;
 		__u64 sector;
 	} u;
-	commit_id_t last_bytes[0];
+	commit_id_t last_bytes[];
 	/* __u8 tag[0]; */
 };
 

drivers/md/dm-log-writes.c

···
 	char *data;
 	u32 datalen;
 	struct list_head list;
-	struct bio_vec vecs[0];
+	struct bio_vec vecs[];
 };
 
 struct per_bio_data {

drivers/md/dm-raid.c

···
 		int mode;
 	} journal_dev;
 
-	struct raid_dev dev[0];
+	struct raid_dev dev[];
 };
 
 static void rs_config_backup(struct raid_set *rs, struct rs_layout *l)

drivers/md/dm-raid1.c

···
 	struct work_struct trigger_event;
 
 	unsigned nr_mirrors;
-	struct mirror mirror[0];
+	struct mirror mirror[];
 };
 
 DECLARE_DM_KCOPYD_THROTTLE_WITH_MODULE_PARM(raid1_resync_throttle,

drivers/md/dm-stats.c

···
 	size_t percpu_alloc_size;
 	size_t histogram_alloc_size;
 	struct dm_stat_percpu *stat_percpu[NR_CPUS];
-	struct dm_stat_shared stat_shared[0];
+	struct dm_stat_shared stat_shared[];
 };
 
 #define STAT_PRECISE_TIMESTAMPS		1

drivers/md/dm-stripe.c

···
 	/* Work struct used for triggering events*/
 	struct work_struct trigger_event;
 
-	struct stripe stripe[0];
+	struct stripe stripe[];
 };
 
 /*

drivers/md/dm-switch.c

···
 	/*
 	 * Array of dm devices to switch between.
 	 */
-	struct switch_path path_list[0];
+	struct switch_path path_list[];
 };
 
 static struct switch_ctx *alloc_switch_ctx(struct dm_target *ti, unsigned nr_paths,

drivers/md/persistent-data/dm-btree-internal.h

···
 
 struct btree_node {
 	struct node_header header;
-	__le64 keys[0];
+	__le64 keys[];
 } __packed;