staging/lustre/llog: MGC to use OSD API for backup logs

MGC uses lvfs API to access local llogs blocking removal of old code

- llog_is_empty() and llog_backup() are introduced

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2059
Lustre-change: http://review.whamcloud.com/5049


Cc: Levente Kurusa <levex@linux.com>
Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
[pick client side change only -- Peng Tao]
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>