RPC: clarify rpc_run_task error handling

rpc_run_task can only fail if it is not passed in a preallocated task.
However, that is not at all clear with the current code.  So
remove several impossible to occur failure checks.

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