ceph: using hash value to compose dentry offset

If MDS sorts dentries in dirfrag in hash order, we use hash value to
compose dentry offset. dentry offset is:

  (0xff << 52) | ((24 bits hash) << 28) |
  (the nth entry hash hash collision)

This offset is stable across directory fragmentation. This alos means
there is no need to reset readdir offset if directory get fragmented
in the middle of readdir.

Signed-off-by: Yan, Zheng <zyan@redhat.com>

include/linux/ceph/ceph_fs.h

@@ -357,6 +357,7 @@ extern const char *ceph_mds_op_name(int op);
  */
 #define CEPH_READDIR_FRAG_END		(1<<0)
 #define CEPH_READDIR_FRAG_COMPLETE	(1<<8)
+#define CEPH_READDIR_HASH_ORDER		(1<<9)
 
 union ceph_mds_request_args {
 	struct {