btrfs-progs: balance: silence compiler warning

cmds-balance.c: In function 'cmd_balance_start':
cmds-balance.c:654:6: warning: ignoring return value of 'chdir', declared with
		attribute warn_unused_result [-Wunused-result]
	chdir("/");

Reported-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

cmds-balance.c

@@ -646,7 +646,12 @@ static int cmd_balance_start(int argc, char **argv)
 				"unable to fork to run balance in background");
 				exit(1);
 			case (0):
-				chdir("/");
+				/*
+				 * Read the return value to silence compiler
+				 * warning. Change to / should succeed and
+				 * we're not in a security-sensitive context.
+				 */
+				i = chdir("/");
 				close(0);
 				close(1);
 				close(2);