Wip 9958


@liewegas . This patchset is the handle about handling fadvise flags on osd side. For filestore, i don't handle CEPH_OSD_OP_FLAG_FADVISE_WILLNEED. Because from the "man posix_fadivse", willneed mean a noblock(background) read. If after willneed and then read, i don't know whether have sense. 
plz add #ifdef HAVE_POSIX_FADVISE to avoid compile error for legacy os or others