wg-quick: linux: only touch net.ipv4 for v4

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

src/wg-quick/linux.bash

@@ -194,9 +194,11 @@ add_default() {
 	cmd ip $proto route add "$1" dev "$INTERFACE" table $table
 	cmd ip $proto rule add not fwmark $table table $table
 	cmd ip $proto rule add table main suppress_prefixlength 0
+	if [[ $proto == -4 ]]; then
 		while read -r key _ value; do
 			[[ $value -eq 1 ]] && sysctl -q "$key=2"
 		done < <(sysctl -a -r '^net\.ipv4.conf\.[^ .=]+\.rp_filter$')
+	fi
 	return 0
 }