LU-6602 obdclass: variable llog chunk size

LU-6602 obdclass: variable llog chunk size

Do not use fix LLOG_CHUNK_SIZE (8192 bytes), and
it will get the llog_chunk_size from llog_log_hdr.
Accordingly llog header will be variable too, so
we can enlarge the bitmap in the header, then
have more records in each llog file.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I1d18c4a502c5ab51d574bb026c94752ddb7dff8a
Reviewed-on: http://review.whamcloud.com/14883
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>