LU-11036 test: race in sanity-lfsck test_8

LU-11036 test: race in sanity-lfsck test_8

In sanity-lfsck test_8, "sleep 1" is run after START_NAMESPACE,
but it still has chance that LFSCK status is complete but LFSCK
thread not quit yet, therefore the following START_NAMESPACE may fail
with -EALREADY. Just check the first lfsck started scanning.

And similarly use wait_update to check flags for DELAY3.

Lustre-change: https://review.whamcloud.com/51720
Lustre-commit: f1ddb4093ed623c1382f75e47807e4081962cc3d

Test-Parameters: trivial MDSCOUNT=2 MDTCOUNT=4 testlist=sanity-lfsck env=ONLY=8,ONLY_REPEAT=10
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ie1f612bebb52c4755e5b4e13d58ab8bf2aeb2832
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51927
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>