Revert "wgengine,net,ipn,disco: split up and define different types of MTU"

This reverts commit 059051c58a.

Signed-off-by: Val <valerie@tailscale.com>

wgengine/magicsock/endpoint.go

@@ -422,10 +422,6 @@ func (de *endpoint) noteActiveLocked() {
 	}
 }
 
-// MaxDiscoPingSize is the largest useful ping message size that we
-// can send - the maximum packet size minus the IPv4 and UDP headers.
-var MaxDiscoPingSize = tstun.MaxPacketSize - 20 - 8
-
 // cliPing starts a ping for the "tailscale ping" command. res is value to call cb with,
 // already partially filled.
 func (de *endpoint) cliPing(res *ipnstate.PingResult, size int, cb func(*ipnstate.PingResult)) {
@@ -437,11 +433,6 @@ func (de *endpoint) cliPing(res *ipnstate.PingResult, size int, cb func(*ipnstat
 		cb(res)
 		return
 	}
-	if size > MaxDiscoPingSize {
-		res.Err = errPingTooBig.Error()
-		cb(res)
-		return
-	}
 
 	now := mono.Now()
 	udpAddr, derpAddr, _ := de.addrForSendLocked(now)
@@ -466,7 +457,6 @@ func (de *endpoint) cliPing(res *ipnstate.PingResult, size int, cb func(*ipnstat
 var (
 	errExpired     = errors.New("peer's node key has expired")
 	errNoUDPOrDERP = errors.New("no UDP or DERP addr")
-	errPingTooBig  = errors.New("ping size too big")
 )
 
 func (de *endpoint) send(buffs [][]byte) error {
@@ -574,9 +564,13 @@ const discoPingSize = len(disco.Magic) + key.DiscoPublicRawLen + disco.NonceLen
 // The caller should use de.discoKey as the discoKey argument.
 // It is passed in so that sendDiscoPing doesn't need to lock de.mu.
 func (de *endpoint) sendDiscoPing(ep netip.AddrPort, discoKey key.DiscoPublic, txid stun.TxID, size int, logLevel discoLogLevel) {
-	size = min(size, MaxDiscoPingSize)
-	padding := max(size-discoPingSize, 0)
-
+	padding := 0
+	if size > int(tstun.DefaultMTU()) {
+		size = int(tstun.DefaultMTU())
+	}
+	if size-discoPingSize > 0 {
+		padding = size - discoPingSize
+	}
 	sent, _ := de.c.sendDiscoMessage(ep, de.publicKey, discoKey, &disco.Ping{
 		TxID:    [12]byte(txid),
 		NodeKey: de.c.publicKeyAtomic.Load(),