staging: lustre: llog: record the minimum record size

The minimum record size will be recorded in llh_size, which is
only used by fixed size record llog now, and also add another
flag LLOG_F_IS_FIXSIZE to indicate the fix size record llog.

Signed-off-by: wang di <di.wang@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-7050
Reviewed-on: http://review.whamcloud.com/16103


Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>