Branch HEAD

b=15797
i=Nikita
i=Vitaly

(1)Replace "la->la_valid" with "ma->ma_attr_flags" to pass
   "MDS_OPEN_OWNEROVERRIDE".
(2)Drop "lu_attr" parameter for xattr_{set,del} method.
   The caller should guarantee to update the object ctime.
(3)MDT can detect whether the "setattr" is SOM case or
   truncate case, it is unnecessary to pass special flag
   ("MDS_ATTR_TRUNC") from client to MDT for truncate case.