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

cmd/mount_zfs/mount_zfs.c

@@ -367,7 +367,7 @@ main(int argc, char **argv)
 				    "mount the filesystem again.\n"), dataset);
 				return (MOUNT_SYSERR);
 			}
-			/* fallthru */
+			fallthrough;
 #endif
 		default:
 			(void) fprintf(stderr, gettext("filesystem "

cmd/zdb/zdb.c

@@ -4564,7 +4564,7 @@ dump_path_impl(objset_t *os, uint64_t obj, char *name)
 	case DMU_OT_DIRECTORY_CONTENTS:
 		if (s != NULL && *(s + 1) != '\0')
 			return (dump_path_impl(os, child_obj, s + 1));
-		/*FALLTHROUGH*/
+		fallthrough;
 	case DMU_OT_PLAIN_FILE_CONTENTS:
 		dump_object(os, child_obj, dump_opt['v'], &header, NULL, 0);
 		return (0);

cmd/zfs/zfs_main.c

@@ -7310,6 +7310,7 @@ unshare_unmount(int op, int argc, char **argv)
 				if (zfs_prop_get_int(zhp, ZFS_PROP_CANMOUNT) ==
 				    ZFS_CANMOUNT_NOAUTO)
 					continue;
+				break;
 			default:
 				break;
 			}