LU-6622 tests: reduce chance of stat timeout in lnet-selftest

LU-6622 tests: reduce chance of stat timeout in lnet-selftest

lst_end_session() calls 'lst show_error' before 'lst stop b' which
stops batched tests. Stat RPC sent by 'lst show_error' may timeout due
to network congestion cause by running batched tests. 'lst show_error'
reports stat RPC's timeout case, so that check_lst_err() detects that
errors were found and assumes that the test failed.
Stop batched tests before call to 'lst show_error' in order to reduce
chance of RCS timeout during 'lst show_error'.

Test-Parameters: trivial
Xyratex-bug-id: MRP-1101
Signed-off-by: Vladimir Saveliev <vladimir.saveliev@seagate.com>
Change-Id: Ibe4fa6399c9ea0b0063ac41494f0725eb14f0e8a
Reviewed-on: http://review.whamcloud.com/14881
Reviewed-by: Doug Oucharek <doug.s.oucharek@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>