fix "stat.Rdev" invalid operation mismatched types on mips64el

compile error the "stat.Rdev" variable and "s.Rdev" mismatched types on mips64el
convert "stat.Rdev" type to uint64 explicitly

Signed-off-by: Xiaodong Liu <liuxiaodong@loongson.cn>

pkg/system/stat_unix_test.go

@@ -31,7 +31,8 @@ func TestFromStatT(t *testing.T) {
 	if stat.Gid != s.GID() {
 		t.Fatal("got invalid gid")
 	}
-	if stat.Rdev != s.Rdev() {
+	//nolint:unconvert // conversion needed to fix mismatch types on mips64el
+	if uint64(stat.Rdev) != s.Rdev() {
 		t.Fatal("got invalid rdev")
 	}
 	if stat.Mtim != s.Mtim() {