LU-506 statahead: revalidating dentry may be stale

The revalidating dentry may be stale (other client recreated this
file), so the statahead inode can be different from dentry inode.
Return -ESTALE instead of LASSERT here.

And for this kind of dentry, it shouldn't be revalidated, but dropped
and re-lookup. To achieve this, if ll_statahead_enter() fails to get
a valid dentry, and the return value is not 0 or -EAGAIN,
ll_revalidate_it() should return 0 directly, and let VFS calls
.lookup to get a new dentry.

Signed-off-by: Lai Siyao <laisiyao@whamcloud.com>
Change-Id: I3eae450069a59d18d138d0cf7538a225e6d6deb8
Reviewed-on: http://review.whamcloud.com/2492
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Peng Tao <tao.peng@emc.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>