LU-19015 llog: logic for skipping a zeroed record

LU-19015 llog: logic for skipping a zeroed record

For ENOSPC errors during dt_write() and threads races, the changelog
could have a sparse file with zeros inside. The current processing
logic skips records for the next chunk.
The patch adds the abilty to skip only zeros in the buffer and start
from a valid record.
Also fix changes the llog_test 8 so that it uses non-zero byte for
corruption.

Fixes: cb1290768df9 ("LU-18218 mdd: changelog specific write function")
Signed-off-by: Alexander Boyko <alexander.boyko@hpe.com>
Change-Id: I7263764ba6a89f226995b8967631eaa6d5bdd4dd
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59267
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Andriy Skulysh <andriy.skulysh@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>