recv() on a closed TCP socket returns ENOTCONN

From RFC 793 s3.9 p58 Event Processing:

If RECEIVE Call arrives in CLOSED state and the user has access to such a
connection, the return should be "error: connection does not exist"

Fixes #1598

PiperOrigin-RevId: 293494287

pkg/sentry/socket/netstack/netstack.go

@@ -2229,11 +2229,16 @@ func (s *SocketOperations) coalescingRead(ctx context.Context, dst usermem.IOSeq
 	var copied int
 
 	// Copy as many views as possible into the user-provided buffer.
-	for dst.NumBytes() != 0 {
+	for {
+		// Always do at least one fetchReadView, even if the number of bytes to
+		// read is 0.
 		err = s.fetchReadView()
 		if err != nil {
 			break
 		}
+		if dst.NumBytes() == 0 {
+			break
+		}
 
 		var n int
 		var e error