SUNRPC: Signalled ASYNC tasks need to exit

Ensure that signalled ASYNC rpc_tasks exit immediately instead of
spinning until a timeout (or forever).

To avoid checking for the signal flag on every scheduler iteration,
the check is instead introduced in the client's finite state
machine.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Fixes: ae67bd38

 ("SUNRPC: Fix up task signalling")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>