Currently, if we restart an RPC call after having set an RPC delay (for
 instance in the case where an NFS EJUKEBOX error has occurred) the call
 to rpc_delete_timer() at the top of the rpc_execute() loop will
 kill off our timer.

 This patch causes rpc_delete_timer() to detect if the rpc_task is still
 queued on a wait queue, and refuse to delete the timer if this is the case.

 Problem diagnosed by Jan Sanislo and Olaf Kirch.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>