UBIFS: introduce list sorting debugging checks

The UBIFS bug in the GC list sorting comparison functions inspired
me to write internal debugging check functions which verify that
the list of nodes is sorted properly.

So, this patch implements 2 new debugging functions:
 o 'dbg_check_data_nodes_order()' - check order of data nodes list
 o 'dbg_check_nondata_nodes_order()' - check order of non-data nodes list

The debugging functions are executed only if general UBIFS debugging checks are
enabled. And they are compiled out if UBIFS debugging is disabled.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>