objectstore: add fadvise_flags operations in
ObjectStore::Transaction::append func.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
Merge pull request #3234 from majianpeng/bug-fix

objectstore: add fadvise_flags operations in ObjectStore::Transaction::append func.

Reviewed-by: Sage Weil <sage@redhat.com>