NFS: only invalidate dentrys that are clearly invalid.

Since commit bafc9b75 ("vfs: More precise tests in d_invalidate")
in v3.18, a return of '0' from ->d_revalidate() will cause the dentry
to be invalidated even if it has filesystems mounted on or it or on a
descendant.  The mounted filesystem is unmounted.

This means we need to be careful not to return 0 unless the directory
referred to truly is invalid.  So -ESTALE or -ENOENT should invalidate
the directory.  Other errors such a -EPERM or -ERESTARTSYS should be
returned from ->d_revalidate() so they are propagated to the caller.

A particular problem can be demonstrated by:

1/ mount an NFS filesystem using NFSv3 on /mnt
2/ mount any other filesystem on /mnt/foo
3/ ls /mnt/foo
4/ turn off network, or otherwise make the server unable to respond
5/ ls /mnt/foo &
6/ cat /proc/$!/stack # note that nfs_lookup_revalidate is in the call stack
7/ kill -9 $! # this results in -ERESTARTSYS being returned
8/ observe that /mnt/foo has been unmounted.

This patch changes nfs_lookup_revalidate() to only treat
  -ESTALE from nfs_lookup_verify_inode() and
  -ESTALE or -ENOENT from ->lookup()
as indicating an invalid inode.  Other errors are returned.

Also nfs_check_inode_attributes() is changed to return -ESTALE rather
than -EIO.  This is consistent with the error returned in similar
circumstances from nfs_update_inode().

As this bug allows any user to unmount a filesystem mounted on an NFS
filesystem, this fix is suitable for stable kernels.

Fixes: bafc9b75

 ("vfs: More precise tests in d_invalidate")
Cc: stable@vger.kernel.org (v3.18+)
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>