UBIFS: introduce debugfs helpers

Separate out pieces of code from the debugfs file read/write functions and
create separate 'interpret_user_input()'/'provide_user_output()' helpers. These
helpers will be needed in one of the following patches, so this is just a
preparational change.

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