EX-8570 lipe: add lpcc sub command to trigger purge scan

EX-8570 lipe: add lpcc sub command to trigger purge scan

Add a sub command 'lpcc purge-scan' to trigger purge
scanning by sending SIGUSR2 to matching lpcc_purge
process.

Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: trivial
Change-Id: I976621fe787daa15b8206eed97efdebe75cd7425
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53036
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>