LU-14688 mdt: changelog purge deletes plain llog

LU-14688 mdt: changelog purge deletes plain llog

With a massive cancel records changelog could delete a plain
llog file and skip one by one record cancelling.
Also patch fixes the race between llog_destroy and llog_next_block.

HPE-bug-id: LUS-9950
Signed-off-by: Alexander Boyko <alexander.boyko@hpe.com>
Change-Id: I47c2ed97945e979745255381f83b6a417d7ba8b1
Reviewed-on: https://review.whamcloud.com/43719
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>