LU-3906 tests: fix space estimation codes in run_compilebench()

While running compilebench test, the actual space required for each
kernel directory is about 1GB instead of 680MB. This patch fixes this
number in run_compilebench() so as to make the space estimation
more correct.

Test-Parameters: envdefinitions=SLOW=yes,ONLY=compilebench \
testlist=parallel-scale,parallel-scale-nfsv3,parallel-scale-nfsv4

Test-Parameters: envdefinitions=SLOW=yes,ONLY=compilebench \
mdtfilesystemtype=zfs mdsfilesystemtype=zfs ostfilesystemtype=zfs \
ostcount=2 mdtcount=1 \
testlist=parallel-scale,parallel-scale-nfsv3,parallel-scale-nfsv4

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I8aa14673b8df99f6b50980ee091acda76da7cd9a
Reviewed-on: http://review.whamcloud.com/8258
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>