incorrect argument checking for "btrfs sub snap -r"

Looks like this was missing in integration-20110626 for the
readonly snapshot patch:

Signed-off-by: Hugo Mills <hugo@carfax.org.uk>

btrfs.c

@@ -49,7 +49,7 @@ static struct Command commands[] = {
 	/*
 		avoid short commands different for the case only
 	*/
-	{ do_clone, 2,
+	{ do_clone, -2,
 	  "subvolume snapshot", "[-r] <source> [<dest>/]<name>\n"
 		"Create a writable/readonly snapshot of the subvolume <source> with\n"
 		"the name <name> in the <dest> directory.",