NFSv4: Don't report revoked delegations as valid in nfs_have_delegation()

If the delegation is revoked, then it can't be used for caching.

Fixes: 869f9dfa ("NFSv4: Fix races between nfs_remove_bad_delegation()...")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: stable@vger.kernel.org # v3.19+
Tested-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>