LU-6968 llog: Only update necessary part of header

LU-6968 llog: Only update necessary part of header

During llog_cancel_rec(), only update the part of header,
instead of the whole header, otherwise it might need pack
the whole header (32KB) into the RPC for each record
cancellation.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I1e6e04b5b725ca8ed500b6c4d767719d7ff5f658
Reviewed-on: http://review.whamcloud.com/15905
Tested-by: Jenkins
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>