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.

Change-Id: I2a9ded4b10ef90a9e02b1b0f1229c87177f046d4
Xyratex-bug-id: MRP-1779
Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-on: http://review.whamcloud.com/9948
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>