nvme: Do not rearm timeout for commands without one.

[mav]

Summary

Admin queue almost always have several ASYNC_EVENT_REQUEST outstanding. They have no timeouts, but their presence in qpair->outstanding_tr caused useless timeout callout rearming twice a second.

While there, relax timeout callout period from 0.5s to 0.5-1s to improve aggregation. Command timeouts are measured in seconds, so we don't need to be precise here.

Test Plan

Without the patch observe periodic callouts for completely idle NVMe SSDs. With the patch confirmed callouts firing only once per queue after I/O command completion.

[imp]

This looks great! Thanks for the improvement. You're right, the timeout is there as a backstop and except for a few admin commands the error will be less than 1%. Even for those admin commands, this won't create a problem since they are infrequent.