LU-4290 llog: discard unavailable records and keep going

LU-4290 llog: discard unavailable records and keep going

if llog can't process some records due to I/O errors or
corruption, just discard them from the header and keep
going. a new test added to verity this behavior.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: Id0dc83ae6239cd55a43eec128b3c750bb9f0894a
Reviewed-on: http://review.whamcloud.com/9281
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>