LU-958 tests: debug_mb set incorrectly for smp or vm

LU-958 tests: debug_mb set incorrectly for smp or vm

For cpus with number of cores or for some VMs,
number of possible CPUs in the system could
be greater than number of cpu reported by getconf.
Added check for maximum debug buffer size.
Added check that "possible" is exist, if not - use old method.

Xyratex-bug-id: MRP-219 incorrect settings for debug_mb
Signed-off-by: Denis Kondratenko <Denis_Kondratenko@xyratex.com>
Change-Id: I1ea367d1b956ae1009c4a501e0f02b6c9209a2f7
Reviewed-on: http://review.whamcloud.com/2377
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>