wgengine/magicsock: restore allocation-free endpoint.DstToString (#5971)

The wireguard-go code unfortunately calls this unconditionally
even when verbose logging is disabled.

Partial revert of #5911.

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

wgengine/magicsock/magicsock.go

@@ -2400,6 +2400,7 @@ func (c *Conn) SetNetworkMap(nm *netmap.NetworkMap) {
 		ep := &endpoint{
 			c:                 c,
 			publicKey:         n.Key,
+			publicKeyHex:      n.Key.UntypedHexString(),
 			sentPing:          map[stun.TxID]sentPing{},
 			endpointState:     map[netip.AddrPort]*endpointState{},
 			heartbeatDisabled: heartbeatDisabled,
@@ -3314,6 +3315,7 @@ type endpoint struct {
 	// These fields are initialized once and never modified.
 	c            *Conn
 	publicKey    key.NodePublic // peer public key (for WireGuard + DERP)
+	publicKeyHex string         // cached output of publicKey.UntypedHexString
 	fakeWGAddr   netip.AddrPort // the UDP address we tell wireguard-go we're using
 
 	// mu protects all following fields.
@@ -3496,7 +3498,7 @@ func (de *endpoint) String() string {
 func (de *endpoint) ClearSrc()           {}
 func (de *endpoint) SrcToString() string { panic("unused") } // unused by wireguard-go
 func (de *endpoint) SrcIP() netip.Addr   { panic("unused") } // unused by wireguard-go
-func (de *endpoint) DstToString() string { return de.publicKey.UntypedHexString() }
+func (de *endpoint) DstToString() string { return de.publicKeyHex }
 func (de *endpoint) DstIP() netip.Addr   { panic("unused") }
 func (de *endpoint) DstToBytes() []byte  { return packIPPort(de.fakeWGAddr) }