Use more conservative locking in NIC.DeliverNetworkPacket

An earlier CL excessively minimizes the period in which it
holds a lock on NIC. This earlier CL had done this out of
the mistaken impression it fixed a broken test, when in
fact it just reduced the rate of failure of a flaky test
in tcp_test.go. This new change holds the lock on NIC
for the duration of the loop over n.endpoints.

PiperOrigin-RevId: 235732487
Change-Id: I53ee6df264f093ddc4d29e9acdcba6b4838cb112

pkg/tcpip/stack/nic.go

@@ -406,14 +406,12 @@ func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, remote, _ tcpip.LinkAddr
 		// n.endpoints is mutex protected so acquire lock.
 		n.mu.RLock()
 		for _, ref := range n.endpoints {
-			n.mu.RUnlock()
 			if ref.protocol == header.IPv4ProtocolNumber && ref.tryIncRef() {
 				r := makeRoute(protocol, dst, src, linkEP.LinkAddress(), ref)
 				r.RemoteLinkAddress = remote
 				ref.ep.HandlePacket(&r, vv)
 				ref.decRef()
 			}
-			n.mu.RLock()
 		}
 		n.mu.RUnlock()
 		return