Rename fallthrough to zfs_fallthrough

Unfortunately macOS has obj-C keyword "fallthrough" in the OS headers.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #13097

cmd/mount_zfs/mount_zfs.c

@@ -390,7 +390,7 @@ main(int argc, char **argv)
 				return (MOUNT_SYSERR);
 			}
 #endif
-			fallthrough;
+			zfs_fallthrough;
 		default:
 			(void) fprintf(stderr, gettext("filesystem "
 			    "'%s' can not be mounted: %s\n"), dataset,

cmd/zdb/zdb.c

@@ -4655,7 +4655,7 @@ dump_path_impl(objset_t *os, uint64_t obj, char *name, uint64_t *retobj)
 	case DMU_OT_DIRECTORY_CONTENTS:
 		if (s != NULL && *(s + 1) != '\0')
 			return (dump_path_impl(os, child_obj, s + 1, retobj));
-		fallthrough;
+		zfs_fallthrough;
 	case DMU_OT_PLAIN_FILE_CONTENTS:
 		if (retobj != NULL) {
 			*retobj = child_obj;

cmd/zpool_influxdb/zpool_influxdb.c

@@ -118,7 +118,7 @@ escape_string(const char *s)
 		case '=':
 		case '\\':
 			*d++ = '\\';
-			fallthrough;
+			zfs_fallthrough;
 		default:
 			*d = *c;
 		}