NFSv4.1: Simplify the sequence setup

Nobody calls nfs4_setup_sequence or nfs41_setup_sequence without
also calling rpc_call_start() on success. This commit therefore
folds the rpc_call_start call into nfs41_setup_sequence().

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