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/zpool_influxdb/zpool_influxdb.c

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