LU-13911 tests: take osc max_rpcs_in_flight limit into account

LU-13911 tests: take osc max_rpcs_in_flight limit into account

max_rpcs_in_flight for osc is limited to 256. sanity.sh:test_115()
tries to set it to ost.OSS.ost_io.threads_started * 4. The test should
make sure that it does not exceed 256.

HPE-bug-id: LUS-5917
Signed-off-by: Vladimir Saveliev <vlaidimir.saveliev@hpe.com>
Change-Id: I38929e1ed0fe7855e7e60ea43742740c01ae1bd8
Reviewed-on: https://review.whamcloud.com/39687
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>