[syserror] Add conversions to linuxerr with temporary Equals method.

Add Equals method to compare syserror and unix.Errno errors to linuxerr errors.
This will facilitate removal of syserror definitions in a followup, and
finding needed conversions from unix.Errno to linuxerr.

PiperOrigin-RevId: 380909667

pkg/sentry/fsimpl/tmpfs/BUILD

@@ -118,6 +118,7 @@ go_test(
     deps = [
         "//pkg/abi/linux",
         "//pkg/context",
+        "//pkg/errors/linuxerr",
         "//pkg/fspath",
         "//pkg/sentry/contexttest",
         "//pkg/sentry/fs/lock",

pkg/sentry/fsimpl/tmpfs/pipe_test.go

@@ -20,6 +20,7 @@ import (
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/errors/linuxerr"
 	"gvisor.dev/gvisor/pkg/fspath"
 	"gvisor.dev/gvisor/pkg/sentry/contexttest"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
@@ -114,7 +115,7 @@ func TestNonblockingWriteError(t *testing.T) {
 	}
 	openOpts := vfs.OpenOptions{Flags: linux.O_WRONLY | linux.O_NONBLOCK}
 	_, err := vfsObj.OpenAt(ctx, creds, &pop, &openOpts)
-	if err != syserror.ENXIO {
+	if !linuxerr.Equals(linuxerr.ENXIO, err) {
 		t.Fatalf("expected ENXIO, but got error: %v", err)
 	}
 }