LU-10577 tests: fix lfsck-performance for separate MGT and MDT

LU-10577 tests: fix lfsck-performance for separate MGT and MDT

lfsck-performance 0,1,2,3 tests run stopall and then mount MDT
which cause the tests failures on configuration with not
combined MGS/MDS.
Patch fixes these tests defects.

Signed-off-by: Elena Gryaznova <c17455@cray.com>
Cray-bug-id: LUS-2534
Test-Parameters: testlist=lfsck-performance
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Andriy Skulysh <c17819@cray.com>
Change-Id: I24c15b9998511bab3dc6fdd3445793e70281c890
Reviewed-on: https://review.whamcloud.com/31075
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>