NFS: Fix nfs_reval_fsid()

mainline: a0356862

We don't need to revalidate the fsid on the root directory. It suffices to
revalidate it on the current directory.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Neil Brown <neilb@suse.de>
CC: Oliver Pinter <oliver.pntr@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>