SUNRPC: rpc_call_null_helper() should set RPC_TASK_SOFT

Clean up.

All of rpc_call_null_helper() call sites assert RPC_TASK_SOFT, so
move that setting into rpc_call_null_helper() itself.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>