[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

thin-provisioning/metadata_checker.h

@@ -48,11 +48,14 @@ namespace thin_provisioning {
 		void set_auto_repair();
 		void set_clear_needs_check();
 
+		// flags for checking
 		bool use_metadata_snap_;
 		data_mapping_options data_mapping_opts_;
 		space_map_options sm_opts_;
 		boost::optional<bcache::block_address> override_mapping_root_;
 		bool ignore_non_fatal_;
+
+		// flags for repairing
 		bool fix_metadata_leaks_;
 		bool clear_needs_check_;
 		bool open_transaction_;