NFS: Fix a sillyrename race...

Ensure that readdir revalidates its data cache after blocking on
sillyrename.

Also fix a typo in nfs_do_call_unlink(): swap the ^= for an |=. The result
is the same, since we've already checked that the flag is unset, but it
makes the code more readable.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>