LU-2843 llog: restore the size of llog for -ENOSPC

the llog is NOT aware its valid size, and if there is some invalid
space in the tail of the llog file, which could be caused if the
free space in the last block(512 bytes) of the llog file can't hold
the next record but it failed to get an extra block for -ENOSPC,
then it will mistake the data written partially for normal llog data!

Change-Id: Ie2619843b538cbb64ae21f9f2a12ff85a5a3e8b4
Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-on: http://review.whamcloud.com/5604
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>