LU-15645 obdclass: llog to handle gaps

LU-15645 obdclass: llog to handle gaps

due to old errors an update llog can contaain gaps in index.
this shouldn't block llog processing and recovery. actual
gaps in transaction sequence should be catched by VBR.

Lustre-change: https://review.whamcloud.com/46837
Lustre-commit: TBD (from b3de0d57bd0f7cd2e918aa9d3f08be1c69697b80)

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I11ec817e356f9658118c34706ef3a533e7faba83
Reviewed-on: https://review.whamcloud.com/46884
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>