btrfs-progs: fi du: update help text

Move the command-specific options to the beginning, add missing long
option.

Signed-off-by: David Sterba <dsterba@suse.com>

cmds-fi-du.c

@@ -526,8 +526,8 @@ out:
 const char * const cmd_filesystem_du_usage[] = {
 	"btrfs filesystem du [options] <path> [<path>..]",
 	"Summarize disk usage of each file.",
+	"-s|--summarize     display only a total for each argument",
 	HELPINFO_UNITS_LONG,
-	"-s                 display only a total for each argument",
 	NULL
 };