LU-6565 llog: Do not update the whole llog header

LU-6565 llog: Do not update the whole llog header

During adding llog record ( see llog_osd_write_rec()),
do not update the whole llog header, but only update
the byte where the bit locates and the counter.

So cross-MDT operation, which needs to write all of
updates of the operation in the llog record, does not
have to write the whole llog header(8K bytes) into
the update record, instead only writing a few bytes.
It will save a lot space for update logs and
ptlrpc request, and help to pack more updates into
the update log.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I777afbb111da6c1bc291735f9c4f105c48b3966b
Reviewed-on: http://review.whamcloud.com/14678
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>