LU-18924 hsm: fix the crash cause by huge max_requests

LU-18924 hsm: fix the crash cause by huge max_requests

Some variables in function mdt_coordinator() should be unsigned
type to avoid memory allocation crash caused by huge parameter
mdt.*.hsm.max_requests.

To avoid such a failure earlier, the sum of MDT max_requests is
limited to 1/8 of total memory.
If it is bigger than this limit, it will be recalculated by this
limit and a useful warning message with memory information and
the limit will be printed.

Also, sanity-hsm.sh test_40 is modified to verify this patch and
stack_trap is added to test_50 and test_100 to restore the default
max_requests value correctly.

Test-Parameters: testlist=sanity-hsm env=ONLY=40,ONLY_REPEAT=20
Test-Parameters: testlist=sanity-hsm
Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Change-Id: I3f6f9722c2af34a4632dc1620ad191774b8ed403