link_path_walk() currently treats the special filenames ".", ".."
and "/" differently in that it does not call down to the filesystem in
order to revalidate the cached dentry, but just assumes that it is
fine.

  For most filesystems this is OK, but it the case of the stateless
NFS, this means that it circumvents path staleness detection, and the
attribute+data cache revalidation code on such common commands as
opendir(".").

This change provides a way to do such revalidation for NFS without
impacting other filesystems. 

Note: the failure to revalidate the path here does not result in a
call to d_invalidate() unlike (all?) other calls to d_revalidate(). It
only results in an ESTALE error being returned to the caller.