wg-quick: anchor sysctl regex to start and end

This doesn't actually fix a real problem, but it is more correct than
not having it.

Suggested-by: Aaron Sigel <aaron@vtty.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

src/wg-quick.bash

@@ -163,7 +163,7 @@ add_default() {
 	local key value
 	while read -r key _ value; do
 		[[ $value -eq 1 ]] && sysctl -q "$key=2"
-	done < <(sysctl -a -r 'net\.ipv4.conf\..+\.rp_filter')
+	done < <(sysctl -a -r '^net\.ipv4.conf\.[^ .=]+\.rp_filter$')
 	return 0
 }