Cleanup: Use C99 flexible array members instead of zero length arrays

The Linux 5.16.14 kernel's coccicheck caught this. The semantic
patch that caught it was:

./scripts/coccinelle/misc/flexible_array.cocci

The Linux kernel's documentation makes a good case for why we should not
use these:

https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes openzfs#14372

module/os/linux/spl/spl-zone.c

@@ -50,7 +50,7 @@ typedef struct zone_datasets {
 typedef struct zone_dataset {
 	struct list_head zd_list;	/* zone_dataset linkage */
 	size_t zd_dsnamelen;		/* length of name */
-	char zd_dsname[0];		/* name of the member dataset */
+	char zd_dsname[];		/* name of the member dataset */
 } zone_dataset_t;
 
 #if defined(CONFIG_USER_NS) && defined(HAVE_USER_NS_COMMON_INUM)

module/os/linux/zfs/vdev_disk.c

@@ -71,7 +71,7 @@ typedef struct dio_request {
 	atomic_t		dr_ref;		/* References */
 	int			dr_error;	/* Bio error */
 	int			dr_bio_count;	/* Count of bio's */
-	struct bio		*dr_bio[0];	/* Attached bio's */
+	struct bio		*dr_bio[];	/* Attached bio's */
 } dio_request_t;
 
 /*

module/zfs/dsl_scan.c

@@ -281,7 +281,7 @@ typedef struct scan_io {
 	 * event of an error. This array must go at the end of the
 	 * struct to allow this for the variable number of elements.
 	 */
-	dva_t			sio_dva[0];
+	dva_t			sio_dva[];
 } scan_io_t;
 
 #define	SIO_SET_OFFSET(sio, x)		DVA_SET_OFFSET(&(sio)->sio_dva[0], x)