nfsd: do not clear rpc_resp in nfsd4_cb_done_sequence

This is incorrect when a callback is has to be restarted, in which case
the XDR decoding of the second iteration will see a NULL cb argument.

[hch: updated description]
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>