main: set default scan mode to mode 4 (EXTENT)

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-usage.txt

@@ -14,7 +14,7 @@ Load management options:
     -g, --loadavg-target  Target load average for worker threads (default none)
 
 Filesystem tree traversal options:
-    -m, --scan-mode       Scanning mode (0..3, default 3)
+    -m, --scan-mode       Scanning mode (0..4, default 4)
 
 Workarounds:
     -a, --workaround-btrfs-send    Workaround for btrfs send

src/bees.cc

@@ -649,7 +649,7 @@ bees_main(int argc, char *argv[])
 	unsigned thread_min = 0;
 	double load_target = 0;
 	bool workaround_btrfs_send = false;
-	BeesRoots::ScanMode root_scan_mode = BeesRoots::SCAN_MODE_INDEPENDENT;
+	BeesRoots::ScanMode root_scan_mode = BeesRoots::SCAN_MODE_EXTENT;
 
 	// Configure getopt_long
 	static const struct option long_options[] = {