NFS: Remove bad delegations during open recovery

I put the client into an open recovery loop by:
	Client: Open file
		read half
	Server: Expire client (echo 0 > /sys/kernel/debug/nfsd/forget_clients)
	Client: Drop vm cache (echo 3 > /proc/sys/vm/drop_caches)
		finish reading file

This causes a loop because the client never updates the nfs4_state after
discovering that the delegation is invalid.  This means it will keep
trying to read using the bad delegation rather than attempting to re-open
the file.
Signed-off-by: NBryan Schumaker <bjschuma@netapp.com>
CC: stable@vger.kernel.org [3.4+]
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4proc.c

@@ -1774,7 +1774,11 @@ static void nfs41_clear_delegation_stateid(struct nfs4_state *state)
 		 * informs us the stateid is unrecognized. */
 		if (status != -NFS4ERR_BAD_STATEID)
 			nfs41_free_stateid(server, stateid);
+		nfs_remove_bad_delegation(state->inode);
 
+		write_seqlock(&state->seqlock);
+		nfs4_stateid_copy(&state->stateid, &state->open_stateid);
+		write_sequnlock(&state->seqlock);
 		clear_bit(NFS_DELEGATED_STATE, &state->flags);
 	}
 }