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.

Xyratex-bug-id: MRP-219 incorrect settings for debug_mb

Reviewed-by: Andrew Perepechko <Andrew_Perepechko@xyratex.com>
Reviewed-by: Alexey Lyashko <Alexey_Lyashko@xyratex.com>
Signed-off-by: Denis Kondratenko <Denis_Kondratenko@xyratex.com>
Change-Id: I7001af7b1c88d5be056734d7d73a0263cca01627
Reviewed-on: http://review.whamcloud.com/1912
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>