Add IRQ guards to critical IPMI response section

This resolves sporadic Zephyr crashes on Blackbird hosts
after the host enters skiboot.

kestrel/src/kestrel.c

@@ -1861,18 +1861,26 @@ static int process_host_to_bmc_ipmi_bt_transactions(void)
             }
             break;
         case 3:
-            // Initialize pointer
-            response_ptr = (ipmi_response_message_t *)(HOSTLPCSLAVE_BASE + AQUILA_LPC_IPMI_BT_DATA_BLOCK_OFFSET);
+            {
+                // Deactivate interrupts on entering critical section
+                int key = irq_lock();
 
-            // Send response
-            // A full copy is done so as to ensure any potentially sensitive data stored
-            // in the IPMI BT buffer from a previous request is overwritten
-            *response_ptr = response;
+                // Initialize pointer
+                response_ptr = (ipmi_response_message_t *)(HOSTLPCSLAVE_BASE + AQUILA_LPC_IPMI_BT_DATA_BLOCK_OFFSET);
 
-            // Signal BMC data ready
-            dword = 0;
-            dword |= (1 << IPMI_BT_CTL_B2H_ATN_SHIFT);
-            write_aquila_register(HOSTLPCSLAVE_BASE, AQUILA_LPC_REG_IPMI_BT_STATUS, dword);
+                // Send response
+                // A full copy is done so as to ensure any potentially sensitive data stored
+                // in the IPMI BT buffer from a previous request is overwritten
+                *response_ptr = response;
+
+                // Signal BMC data ready
+                dword = 0;
+                dword |= (1 << IPMI_BT_CTL_B2H_ATN_SHIFT);
+                write_aquila_register(HOSTLPCSLAVE_BASE, AQUILA_LPC_REG_IPMI_BT_STATUS, dword);
+
+                // Re-activate interupts on exiting critical section
+                irq_unlock(key);
+            }
 
             ipmi_bt_transaction_state = 4;
             break;