[thin_check] Allow using --clear-needs-check and --skip-mappings together

Although it is not recommended to clear the flag without a full
examination, however, the usage has been documented as an approach
to reduce lvchange run time [1]. For the purpose of backward
compatibility and avoiding boot failure after upgrading thin_check [2],
the limitation is now removed.

[1] https://wiki.archlinux.org/index.php/LVM#Thinly-provisioned_root_volume_device_times_out
[2] Community feedback on previous commit:
    https://github.com/jthornber/thin-provisioning-tools/commit/b278f4f

tests/thin_check.rs

@@ -166,11 +166,6 @@ fn clear_needs_check_incompatible_opts() -> Result<()> {
         "--super-block-only",
         &md
     ))?;
-    run_fail(thin_check!(
-        "--clear-needs-check-flag",
-        "--skip-mappings",
-        &md
-    ))?;
     run_fail(thin_check!(
         "--clear-needs-check-flag",
         "--ignore-non-fatal-errors",
@@ -204,6 +199,19 @@ fn no_clear_needs_check_if_error() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn clear_needs_check_if_skip_mappings() -> Result<()> {
+    let mut td = TestDir::new()?;
+    let md = prep_metadata(&mut td)?;
+    set_needs_check(&md)?;
+    generate_metadata_leaks(&md, 1, 0, 1)?;
+
+    assert!(get_needs_check(&md)?);
+    thin_check!("--clear-needs-check-flag", "--skip-mappings", &md).run()?;
+    assert!(!get_needs_check(&md)?);
+    Ok(())
+}
+
 #[test]
 fn metadata_leaks_are_non_fatal() -> Result<()> {
     let mut td = TestDir::new()?;