UBIFS: remove incorrect index space check

When we report free space to user-space, we should not report
0 if the amount of empty LEBs is too low, because they would
be produced by GC when needed. Thus, just call
'ubifs_calc_available()' straight away which would take
'min_idx_lebs' into account anyway.

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