Use fallthrough macro

As of the Linux 5.9 kernel a fallthrough macro has been added which
should be used to anotate all intentional fallthrough paths.  Once
all of the kernel code paths have been updated to use fallthrough
the -Wimplicit-fallthrough option will because the default.  To
avoid warnings in the OpenZFS code base when this happens apply
the fallthrough macro.

Additional reading: https://lwn.net/Articles/794944/

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12441

module/zfs/abd.c

@@ -1064,10 +1064,10 @@ abd_raidz_gen_iterate(abd_t **cabds, abd_t *dabd,
 		switch (parity) {
 			case 3:
 				len = MIN(caiters[2].iter_mapsize, len);
-				/* falls through */
+				fallthrough;
 			case 2:
 				len = MIN(caiters[1].iter_mapsize, len);
-				/* falls through */
+				fallthrough;
 			case 1:
 				len = MIN(caiters[0].iter_mapsize, len);
 		}
@@ -1177,11 +1177,11 @@ abd_raidz_rec_iterate(abd_t **cabds, abd_t **tabds,
 			case 3:
 				len = MIN(xiters[2].iter_mapsize, len);
 				len = MIN(citers[2].iter_mapsize, len);
-				/* falls through */
+				fallthrough;
 			case 2:
 				len = MIN(xiters[1].iter_mapsize, len);
 				len = MIN(citers[1].iter_mapsize, len);
-				/* falls through */
+				fallthrough;
 			case 1:
 				len = MIN(xiters[0].iter_mapsize, len);
 				len = MIN(citers[0].iter_mapsize, len);

module/zfs/dsl_prop.c

@@ -749,7 +749,7 @@ dsl_prop_set_sync_impl(dsl_dataset_t *ds, const char *propname,
 		ASSERT(err == 0 || err == ENOENT);
 		err = zap_remove(mos, zapobj, inheritstr, tx);
 		ASSERT(err == 0 || err == ENOENT);
-		/* FALLTHRU */
+		fallthrough;
 	case (ZPROP_SRC_NONE | ZPROP_SRC_RECEIVED):
 		/*
 		 * remove propname$recvd

module/zfs/spa.c

@@ -9533,7 +9533,7 @@ spa_activity_in_progress(spa_t *spa, zpool_wait_activity_t activity,
 	case ZPOOL_WAIT_RESILVER:
 		if ((*in_progress = vdev_rebuild_active(spa->spa_root_vdev)))
 			break;
-		/* fall through */
+		fallthrough;
 	case ZPOOL_WAIT_SCRUB:
 	{
 		boolean_t scanning, paused, is_scrub;

module/zfs/vdev_label.c

@@ -1288,7 +1288,7 @@ vdev_label_read_bootenv(vdev_t *rvd, nvlist_t *bootenv)
 				nvlist_free(config);
 				break;
 			}
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			/* Check for FreeBSD zfs bootonce command string */
 			buf = abd_to_buf(abd);

module/zfs/vdev_raidz_math_scalar.c

@@ -142,7 +142,7 @@ static const struct {
 		a.b[6] = mul_lt[a.b[6]];				\
 		a.b[5] = mul_lt[a.b[5]];				\
 		a.b[4] = mul_lt[a.b[4]];				\
-		/* falls through */					\
+		fallthrough;						\
 	case 4:								\
 		a.b[3] = mul_lt[a.b[3]];				\
 		a.b[2] = mul_lt[a.b[2]];				\

module/zfs/zfs_replay.c

@@ -362,7 +362,7 @@ zfs_replay_create_acl(void *arg1, void *arg2, boolean_t byteswap)
 		zfsvfs->z_fuid_replay = zfs_replay_fuids(fuidstart,
 		    (void *)&name, lracl->lr_fuidcnt, lracl->lr_domcnt,
 		    lr->lr_uid, lr->lr_gid);
-		/*FALLTHROUGH*/
+		fallthrough;
 	case TX_CREATE_ACL_ATTR:
 		if (name == NULL) {
 			lrattr = (lr_attr_t *)(caddr_t)(lracl + 1);
@@ -394,7 +394,7 @@ zfs_replay_create_acl(void *arg1, void *arg2, boolean_t byteswap)
 		zfsvfs->z_fuid_replay = zfs_replay_fuids(fuidstart,
 		    (void *)&name, lracl->lr_fuidcnt, lracl->lr_domcnt,
 		    lr->lr_uid, lr->lr_gid);
-		/*FALLTHROUGH*/
+		fallthrough;
 	case TX_MKDIR_ACL_ATTR:
 		if (name == NULL) {
 			lrattr = (lr_attr_t *)(caddr_t)(lracl + 1);
@@ -519,8 +519,7 @@ zfs_replay_create(void *arg1, void *arg2, boolean_t byteswap)
 		    zfs_replay_fuid_domain(start, &start,
 		    lr->lr_uid, lr->lr_gid);
 		name = (char *)start;
-
-		/*FALLTHROUGH*/
+		fallthrough;
 	case TX_CREATE:
 		if (name == NULL)
 			name = (char *)start;
@@ -537,8 +536,7 @@ zfs_replay_create(void *arg1, void *arg2, boolean_t byteswap)
 		    zfs_replay_fuid_domain(start, &start,
 		    lr->lr_uid, lr->lr_gid);
 		name = (char *)start;
-
-		/*FALLTHROUGH*/
+		fallthrough;
 	case TX_MKDIR:
 		if (name == NULL)
 			name = (char *)(lr + 1);

module/zfs/zio_compress.c

@@ -214,7 +214,7 @@ zio_compress_to_feature(enum zio_compress comp)
 	case ZIO_COMPRESS_ZSTD:
 		return (SPA_FEATURE_ZSTD_COMPRESS);
 	default:
-		/* fallthru */;
+		break;
 	}
 	return (SPA_FEATURE_NONE);
 }