UBIFS: fix the dark space calculation

The dark space calculation should be 64 bit type-casted, when
assigning to tmp64 (similar to how total_free is calculated).
Overflow will occur for very large flashes.

Signed-off-by: srimugunthan <srimugunthan.dhandapani@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>

diff --cc fs/ubifs/recovery.c
Simple merge

diff --cc fs/ubifs/sb.c
Simple merge