NFSv4.1: Clean up nfs4_setup_sequence

Firstly, there is little point in first zeroing out the entire struct
nfs4_sequence_res, and then initialising all fields save one. Just
initialise the last field to zero...

Secondly, nfs41_setup_sequence() has only 2 possible return values: 0, or
-EAGAIN, so there is no 'terminate rpc task' case.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>