dm raid: fix some spelling mistakes in comments

Signed-off-by: Yu Zhe <yuzhe@nfschina.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-raid.c

@@ -1988,7 +1988,7 @@ struct dm_raid_superblock {
 	__le64 sectors; /* Used device size in sectors */
 
 	/*
-	 * Additonal Bit field of devices indicating failures to support
+	 * Additional Bit field of devices indicating failures to support
 	 * up to 256 devices with the 1.9.0 on-disk metadata format
 	 */
 	__le64 extended_failed_devices[DISKS_ARRAY_ELEMS - 1];
@@ -2855,7 +2855,7 @@ static int rs_setup_reshape(struct raid_set *rs)
 	 *
 	 * - in case of adding disk(s), array size has
 	 *   to grow after the disk adding reshape,
-	 *   which'll hapen in the event handler;
+	 *   which'll happen in the event handler;
 	 *   reshape will happen forward, so space has to
 	 *   be available at the beginning of each disk
 	 *
@@ -3148,7 +3148,7 @@ static int raid_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 		 * If a takeover is needed, userspace sets any additional
 		 * devices to rebuild and we can check for a valid request here.
 		 *
-		 * If acceptible, set the level to the new requested
+		 * If acceptable, set the level to the new requested
 		 * one, prohibit requesting recovery, allow the raid
 		 * set to run and store superblocks during resume.
 		 */