nvmf: Narrow scope of sim lock in nvmf_sim_io

nvmf_submit_request() handles races with concurrent queue pair
destruction (or the queue pair being destroyed between
nvmf_allocate_request and nvmf_submit_request), so the lock is not
needed here.  This avoids holding the lock across transport-specific
logic such as queueing mbufs for PDUs to a socket buffer, etc.

Holding the lock across nvmf_allocate_request() ensures that the queue
pair pointers in the softc are still valid as shutdown attempts will
block on the lock before destroying the queue pairs.

Sponsored by:	Chelsio Communications

sys/dev/nvmf/host/nvmf_sim.c

@@ -125,8 +125,8 @@ nvmf_sim_io(struct nvmf_softc *sc, union ccb *ccb)
 		qp = sc->admin;
 	req = nvmf_allocate_request(qp, &nvmeio->cmd, nvmf_ccb_complete,
 	    ccb, M_NOWAIT);
+	mtx_unlock(&sc->sim_mtx);
 	if (req == NULL) {
-		mtx_unlock(&sc->sim_mtx);
 		nvmeio->ccb_h.status = CAM_RESRC_UNAVAIL;
 		xpt_done(ccb);
 		return;
@@ -150,7 +150,6 @@ nvmf_sim_io(struct nvmf_softc *sc, union ccb *ccb)
 	    ("%s: incoming CCB is not in-progress", __func__));
 	ccb->ccb_h.status |= CAM_SIM_QUEUED;
 	nvmf_submit_request(req);
-	mtx_unlock(&sc->sim_mtx);
 }
 
 static void