Add a brief comment explaining why we can return ETIMEDOUT from the call to the

polled interface. Normally this would have the potential to corrupt stack memory
because the completion routines would run after we return. In this case,
however, we're doing a dump so it's safe for reasons explained in the comment.

sys/dev/nvme/nvme_ns_cmd.c

@@ -191,6 +191,14 @@ nvme_ns_dump(struct nvme_namespace *ns, void *virt, off_t offset, size_t len)
 		nvme_qpair_process_completions(req->qpair);
 	}
 
+	/*
+	 * Normally, when using the polling interface, we can't return a
+	 * timeout error because we don't know when the completion routines
+	 * will be called if the command later completes. However, in this
+	 * case we're running a system dump, so all interrupts are turned
+	 * off, the scheduler isn't running so there's nothing to complete
+	 * the transaction.
+	 */
 	if (status.done == FALSE)
 		return (ETIMEDOUT);