EX-5419 lipe: retry to get lpcc_purge stats file

EX-5419 lipe: retry to get lpcc_purge stats file

Sleep and retry to get lpcc_purge stats file after sending signal
Correct statistics data algorithm.

Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: clientdistro=el8.5 testlist=sanity-pcc env=ONLY=210
Change-Id: I79e4b7de871f8ffd290e0c30da4780265fa7e9fa
Reviewed-on: https://review.whamcloud.com/47729
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>