p2p/dnsdisc: fix flaw in dns size calculation (#22533)

This fixes the calculation of the tree branch factor. With the new
formula, we now creat at most 13 children instead of 30, ensuring
the TXT record size will be below 370 bytes.

p2p/dnsdisc/tree.go

@@ -113,10 +113,41 @@ func (t *Tree) Nodes() []*enode.Node {
 	return nodes
 }
 
+/*
+We want to keep the UDP size below 512 bytes. The UDP size is roughly:
+UDP length = 8 + UDP payload length ( 229 )
+UPD Payload length:
+ - dns.id 2
+ - dns.flags 2
+ - dns.count.queries 2
+ - dns.count.answers 2
+ - dns.count.auth_rr 2
+ - dns.count.add_rr 2
+ - queries (query-size + 6)
+ - answers :
+ 	- dns.resp.name 2
+ 	- dns.resp.type 2
+ 	- dns.resp.class 2
+ 	- dns.resp.ttl 4
+ 	- dns.resp.len 2
+ 	- dns.txt.length 1
+ 	- dns.txt resp_data_size
+
+So the total size is roughly a fixed overhead of `39`, and the size of the
+query (domain name) and response.
+The query size is, for example, FVY6INQ6LZ33WLCHO3BPR3FH6Y.snap.mainnet.ethdisco.net (52)
+
+We also have some static data in the response, such as `enrtree-branch:`, and potentially
+splitting the response up with `" "`, leaving us with a size of roughly `400` that we need
+to stay below.
+
+The number `370` is used to have some margin for extra overhead (for example, the dns query
+may be larger - more subdomains).
+*/
 const (
-	hashAbbrev    = 16
-	maxChildren   = 300 / hashAbbrev * (13 / 8)
-	minHashLength = 12
+	hashAbbrevSize = 1 + 16*13/8          // Size of an encoded hash (plus comma)
+	maxChildren    = 370 / hashAbbrevSize // 13 children
+	minHashLength  = 12
 )
 
 // MakeTree creates a tree containing the given nodes and links.