LU-4904 test: write_append_truncate failed with divide error

LU-4904 test: write_append_truncate failed with divide error

fix trunc_size calculation in case of append_size == 1
Don't allow to specify write_max and append_max < 2.

This patch is back-ported from the following one:
Lustre-commit: 85b037f5da6a022919783ef9a87ed6d2a4d102d2
Lustre-change: http://review.whamcloud.com/9948

Test-Parameters: alwaysuploadlogs \
envdefinitions=SLOW=yes,ENABLE_QUOTA=yes,ONLY=write_append_truncate \
mdtfilesystemtype=ldiskfs mdsfilesystemtype=ldiskfs ostfilesystemtype=ldiskfs \
mdtcount=4 mdssizegb=2 ostcount=7 ostsizegb=2 \
testlist=parallel-scale

Xyratex-bug-id: MRP-1779

Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Change-Id: Ia2cf141b3cc78e4f1488a96fc602b78b16869678
Reviewed-on: http://review.whamcloud.com/12358
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>