Do not read-lock NIC recursively

A deadlock may occur if a write lock on a RWMutex is blocked between
nested read lock attempts as the inner read lock attempt will be
blocked in this scenario.

Example (T1 and T2 are differnt goroutines):
  T1: obtain read-lock
  T2: attempt write-lock (blocks)
  T1: attempt inner/nested read-lock (blocks)

Here we can see that T1 and T2 are deadlocked.

Tests: Existing tests pass.
PiperOrigin-RevId: 298426678

pkg/tcpip/stack/nic.go

@@ -451,7 +451,7 @@ func (n *NIC) primaryIPv6Endpoint(remoteAddr tcpip.Address) *referencedNetworkEn
 	cs := make([]ipv6AddrCandidate, 0, len(primaryAddrs))
 	for _, r := range primaryAddrs {
 		// If r is not valid for outgoing connections, it is not a valid endpoint.
-		if !r.isValidForOutgoing() {
+		if !r.isValidForOutgoingRLocked() {
 			continue
 		}