LU-3906 tests: use df instead of lfs_df in run_compilebench()

LU-3906 tests: use df instead of lfs_df in run_compilebench()

Since run_compilebench() will be run on NFS client which has no
Lustre filesystem, we need change to use df instead of lfs_df to
get the space usage information.

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

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: Ia1a9586586b888231952830180157a9e37f08843
Reviewed-on: http://review.whamcloud.com/8429
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>