LU-15000 llog: read canceled records in llog_backup

LU-15000 llog: read canceled records in llog_backup

llog_backup() do not reproduce index "holes" in the generated copy.
This could result to a llog copy indexes different from the source.
Then it might confuse the configuration update mechanism that rely on
indexes between the MGS source and the target copy.

This index gaps can be caused by "lctl --device MGS llog_cancel".

This patch add "raw" read mode to llog_process* to read canceled
records. So now llog_backup is able to reproduce an exact copy of
the original.

Lustre-change: https://review.whamcloud.com/46552
Lustre-commit: d8e2723b4e9409954846939026c599b0b1170e6e

Signed-off-by: Etienne AUJAMES <etienne.aujames@cea.fr>
Change-Id: I811e23de8f4545bed36a44fedc2638d7418830dd
Reviewed-by: Dominique Martinet <qhufhnrynczannqp.f@noclue.notk.org>
Reviewed-by: DELBARY Gael <gael.delbary@cea.fr>
Reviewed-by: Stephane Thiell <sthiell@stanford.edu>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/48894
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>