l2cap/coc: make coc_continue_tx() thread safe

ble_l2cap_coc_continue_tx() is executed in different tasks: it is
(i) called from ble_l2cap_coc_send() in one or more different task
contexts from user application(s) and (ii) called from
ble_l2cap_coc_le_credits_update() from the NimBLE host task.
Typically, the host task has a higher priority then the user tasks,
so that ble_l2cap_coc_continue_tx() can be interrupted by itself.
This leads to potentially inconsistent state for
`chan->coc_tx->credits` and `chan->coc_tx->data_offset`, consequently
leading to dropped packets.
This commit fixes this by putting all the code code working on these
variables into a critical section.

nimble/host/src/ble_l2cap_coc.c

@@ -403,9 +403,12 @@ ble_l2cap_coc_continue_tx(struct ble_l2cap_chan *chan)
     uint16_t sdu_size_offset;
     int rc;
 
+    ble_hs_lock();
+
     /* If there is no data to send, just return success */
     tx = &chan->coc_tx;
     if (!tx->sdu) {
+        ble_hs_unlock();
         return 0;
     }
 
@@ -458,10 +461,8 @@ ble_l2cap_coc_continue_tx(struct ble_l2cap_chan *chan)
             goto failed;
         }
 
-        ble_hs_lock();
         conn = ble_hs_conn_find_assert(chan->conn_handle);
         rc = ble_l2cap_tx(conn, chan, txom);
-        ble_hs_unlock();
 
         if (rc) {
           /* txom is consumed by l2cap */
@@ -491,14 +492,18 @@ ble_l2cap_coc_continue_tx(struct ble_l2cap_chan *chan)
     if (tx->sdu) {
         /* Not complete SDU sent, wait for credits */
         tx->flags |= BLE_L2CAP_COC_FLAG_STALLED;
+        ble_hs_unlock();
         return BLE_HS_ESTALLED;
     }
 
+    ble_hs_unlock();
     return 0;
 
 failed:
     os_mbuf_free_chain(tx->sdu);
     tx->sdu = NULL;
+    ble_hs_unlock();
+
     os_mbuf_free_chain(txom);
     if (tx->flags & BLE_L2CAP_COC_FLAG_STALLED) {
         ble_l2cap_event_coc_unstalled(chan, rc);