A problem of miscalculated in doSpaceCheck

:Release Notes:
fixed calculation of avalable space in doSpaceCheck()

:Detailed Notes:
If available size calculates incorrectly, space alarm level does not change
and scheduledSpaceCheck also does not call purge().
So, fixed to calculate available size correctly.

:Testing Performed:
Compiled and tested localy.

:QA Notes:

:Issues Addressed:
[ANG-811] [DB] a problem of miscalculated in doSpaceCheck

Open-webOS-DCO-1.0-Signed-off-by: kyouhyun.hwang <kyouhyun.hwang@lge.com>

Change-Id: Id8794a02c5236e72d1fb4085bbdac400e4c26413
Reviewed-on: https://g2g.palm.com/4492
Reviewed-by: DCO Verification
Reviewed-by: Oleksii Uratkin <oleksii.uratkin@lge.com>
Tested-by: Oleksii Uratkin <oleksii.uratkin@lge.com>