unix: fix several staticcheck issues

Resolves a number of issues, except for the clen function being
unused on Linux, as that would require a larger refactor which
probably isn't strictly necessary.

$ staticcheck .
affinity_linux.go:94:8: const m3 is unused (U1000)
affinity_linux.go:95:8: const m4 is unused (U1000)
sendfile_test.go:41:2: the goroutine calls T.Fatal, which must be called in the same goroutine as the test (SA2002)
sendfile_test.go:47:6: this value of err is never used (SA4006)
syscall_linux_test.go:519:21: should use time.Since instead of time.Now().Sub (S1012)
syscall_linux_test.go:530:21: should use time.Since instead of time.Now().Sub (S1012)
syscall_test.go:66:2: this value of ts is never used (SA4006)
syscall_test.go:67:2: this value of ts is never used (SA4006)
syscall_unix.go:88:6: func clen is unused (U1000)

Change-Id: I2611b4559339cea758b5da27ea1f36fb8cc2df3f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/188037
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_test.go

@@ -62,13 +62,10 @@ func TestUname(t *testing.T) {
 // Test that this compiles. (Issue #31735)
 func TestStatFieldNames(t *testing.T) {
 	var st unix.Stat_t
-	var ts *unix.Timespec
-	ts = &st.Atim
-	ts = &st.Mtim
-	ts = &st.Ctim
-	_ = ts
-	secs := int64(st.Mtim.Sec)
-	nsecs := int64(st.Mtim.Nsec)
-	_ = secs
-	_ = nsecs
+	var _ *unix.Timespec
+	_ = &st.Atim
+	_ = &st.Mtim
+	_ = &st.Ctim
+	_ = int64(st.Mtim.Sec)
+	_ = int64(st.Mtim.Nsec)
 }