UBI: change the interface of a debugging check function

This is a minor preparational patch which changes the
'paranoid_check_in_wl_tree()' function interface by adding the 'ubi' parameter
which will be needed there in the next patch.

And while on it, add "const" qualifier to the 'ubi' parameter of the
'paranoid_check_in_pq()' function.

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