UBIFS: introduce lsave debugging

Try to improve UBIFS testing coverage by randomly picking LEBs to
store in lsave, rather than picking them optimally. Create a debugging
version of 'populate_lsave()' for these purposes and enable it when
general debugging self-checks are enabled.

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