wg-quick: look up existing routes properly

This was never really correct, and then 5.1 broke it entirely.

Reported-by: piraty1@inbox.ru
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

src/wg-quick/linux.bash

@@ -164,7 +164,7 @@ add_route() {
 	elif [[ $1 == */0 ]]; then
 		add_default "$1"
 	else
-		[[ $(ip route get "$1" 2>/dev/null) == *dev\ $INTERFACE\ * ]] || cmd ip route add "$1" dev "$INTERFACE"
+		[[ -n $(ip route show dev "$INTERFACE" match "$1" 2>/dev/null) ]] || cmd ip route add "$1" dev "$INTERFACE"
 	fi
 }