Branch b1_6

b=15658
i=huanghua
i=tappro

Support directory read ahead under patchless client mode.
When "ls" a dentry, the system trigger more than once "revalidate" or
"lookup", for "getattr", for "getxattr", and maybe for others.
Under patchless client mode, the operation intent is not accurate,
it maybe misguide the statahead thread. For example:
The "revalidate" call for "getattr" and "getxattr" of a dentry maybe
have the same operation intent -- "IT_GETATTR".
In fact, one dentry should has only one chance to interact with the
statahead thread, otherwise the statahead windows will be confused.