LU-13282 tests: wait $LFSCK_BG_PID properly

LU-13282 tests: wait $LFSCK_BG_PID properly

ha.sh fails with:
  wait: pid $LFSCK_BG_PID is not a child of this shell
because wait $LFSCK_BG_PID duplicates wait in ha_stop_loads().

Patch fixes this issue.

Test-Parameters: trivial
Cray-bug-id: LUS-7930
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Vladimir Saveliev <vladimir.saveliev@hpe.com>
Change-Id: Ifdeab0dc570e9da889ccb34c0b47473e1077bfdc
Reviewed-on: https://review.whamcloud.com/37668
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Alexander Boyko <c17825@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>