nfs41: verify CB_SEQUENCE position in callback compound

CB_SEQUENCE must appear first in the callback compound RPC.
If it is not the first operation NFS4ERR_SEQUENCE_POS must be returned.
If the first operation ni the CB_COMPOUND is not CB_SEQUENCE then
NFS4ERR_OP_NOT_IN_SESSION must be returned.

Signed-off-by: Ricardo Labiaga <ricardo.labiaga@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[nfs41: refactor op preprocessing out of process_op]
Signed-off-by: Benny Halevy <bhalevy@panasas.com>