tcpip: ensure fdbased/mmap_stub.go uses a stateify-recognized suffix

Our Bazel build infrastructure explicitly recognizes some suffixes that can
appear at the end of filenames (tools/bazeldefs/tags.bzl). It uses these tags
to group files passed to go_stateify (tools/defs.bzl:go_library() =>
calculate_sets()).

Each invocation of go_stateify infers build tags from its input files. Before
this CL, all Go files in pkg/tcpip/link/fdbased (except for fdbased_unsafe.go)
are grouped together to generate fdbased_state_autogen.go, so go_stateify
infers builds tags "!linux !amd64,!arm64" (from mmap_stub.go) and "linux ..."
(from most other files); these contradictory build tags mean that
fdbased_state_autogen.go is never actually compiled, preventing its types from
being registered. After this CL, mmap_nonlinux.go independently generates
fdbased_nonlinux_state_autogen.go.

Fixing this causes fdbased_state_autogen.go to be included for the first time,
exposing a second problem:

```
pkg/tcpip/link/fdbased/endpoint_test.go:80:6: context already declared through import of package context ("context")
        .../pkg/tcpip/link/fdbased/fdbased_state_autogen.go:12:2: other declaration of context
```

(Apparently Go imports are a pessimal hybrid of "shared between files in a
package" and "not shared between files in a package".) To fix this, rename the
test type to testContext.

PiperOrigin-RevId: 663522480

pkg/tcpip/link/fdbased/BUILD

@@ -11,7 +11,7 @@ go_library(
         "endpoint.go",
         "endpoint_unsafe.go",
         "mmap.go",
-        "mmap_stub.go",
+        "mmap_nonlinux.go",
         "mmap_unsafe.go",
         "packet_dispatchers.go",
         "processors.go",

pkg/tcpip/link/fdbased/endpoint_test.go

@@ -77,7 +77,7 @@ func checkPacketInfoEqual(t *testing.T, got, want packetInfo) {
 	}
 }
 
-type context struct {
+type testContext struct {
 	t        *testing.T
 	readFDs  []int
 	writeFDs []int
@@ -86,7 +86,7 @@ type context struct {
 	done     chan struct{}
 }
 
-func newContext(t *testing.T, opt *Options) *context {
+func newContext(t *testing.T, opt *Options) *testContext {
 	firstFDPair, err := unix.Socketpair(unix.AF_UNIX, unix.SOCK_SEQPACKET, 0)
 	if err != nil {
 		t.Fatalf("Socketpair failed: %v", err)
@@ -107,7 +107,7 @@ func newContext(t *testing.T, opt *Options) *context {
 		t.Fatalf("Failed to create FD endpoint: %v", err)
 	}
 
-	c := &context{
+	c := &testContext{
 		t:        t,
 		readFDs:  []int{firstFDPair[0], secondFDPair[0]},
 		writeFDs: opt.FDs,
@@ -121,7 +121,7 @@ func newContext(t *testing.T, opt *Options) *context {
 	return c
 }
 
-func (c *context) cleanup() {
+func (c *testContext) cleanup() {
 	for _, fd := range c.readFDs {
 		unix.Close(fd)
 	}
@@ -132,12 +132,12 @@ func (c *context) cleanup() {
 	}
 }
 
-func (c *context) DeliverNetworkPacket(protocol tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) {
+func (c *testContext) DeliverNetworkPacket(protocol tcpip.NetworkProtocolNumber, pkt *stack.PacketBuffer) {
 	pkt.IncRef()
 	c.ch <- packetInfo{protocol, pkt}
 }
 
-func (c *context) DeliverLinkPacket(tcpip.NetworkProtocolNumber, *stack.PacketBuffer) {
+func (c *testContext) DeliverLinkPacket(tcpip.NetworkProtocolNumber, *stack.PacketBuffer) {
 	c.t.Fatal("DeliverLinkPacket not implemented")
 }
 

tools/bazeldefs/tags.bzl

@@ -41,6 +41,7 @@ archs = [
 
 oses = [
     "_linux",
+    "_nonlinux",
 ]
 
 generic = [