SUNRPC: Move remaining RPC client related task initialisation into clnt.c

Now that rpc_run_task() is the sole entry point for RPC calls, we can move
the remaining rpc_client-related initialisation of struct rpc_task from
sched.c into clnt.c.

Also move rpc_killall_tasks() into the same file, since that too is
relative to the rpc_clnt.

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