NFSD: fix decode_cb_sequence4resok

Fix bug introduced in patch
85a56480 NFSD: Update XDR decoders in NFSv4 callback client

Although decode_cb_sequence4resok ignores highest slotid and target highest slotid
it must account for their space in their xdr stream when calling xdr_inline_decode

Cc: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>