This is a resend of the NFS lookup code rewrite, but with the open(".")
VFS fix removed. (I'll resend the 'uses d_revalidate()' version
separately after a suitable delay to allow for comments.)

  Issues fixed by this patch:

 - Use the directory mtime in order to give us a hint when we should
   check for namespace changes.

 - Add support for the 'nocto' flag, in order to turn off the strict
   attribute cache revalidation on file open().

 - Simplify inode lookup. Don't check the 'fsid' field (which appears
   to be buggy in too many servers in order to be reliable). Instead
   we only rely on the inode number (a.k.a. 'fileid') and the
   (supposedly unique) filehandle.