nimble/ll: Fix possible race on TX queue

Removing from ll_tx_pkt_q should be done in critical section since
insertion can be done from isr.

There's no need to protect peek on 1st element since we only insert
in isr and remove in LL so there's no risk of element being removed
after we checked not-null.

nimble/controller/src/ble_ll.c

@@ -711,6 +711,7 @@ ble_ll_tx_pkt_in(void)
     uint16_t pb;
     struct os_mbuf_pkthdr *pkthdr;
     struct os_mbuf *om;
+    os_sr_t sr;
 
     /* Drain all packets off the queue */
     while (STAILQ_FIRST(&g_ble_ll_data.ll_tx_pkt_q)) {
@@ -719,7 +720,9 @@ ble_ll_tx_pkt_in(void)
         om = (struct os_mbuf *)((uint8_t *)pkthdr - sizeof(struct os_mbuf));
 
         /* Remove from queue */
+        OS_ENTER_CRITICAL(sr);
         STAILQ_REMOVE_HEAD(&g_ble_ll_data.ll_tx_pkt_q, omp_next);
+        OS_EXIT_CRITICAL(sr);
 
         /* Strip HCI ACL header to get handle and length */
         handle = get_le16(om->om_data);