InternetSocket: better protect _callback

sigio callbacks can be triggered from interrupt, so changing _callback
needs critical section protection, not just a mutex.

Author: kjbracey

UNITTESTS/features/netsocket/DTLSSocket/unittest.cmake

@@ -22,6 +22,7 @@ set(unittest-test-sources
   features/netsocket/DTLSSocket/test_DTLSSocket.cpp
   stubs/Mutex_stub.cpp
   stubs/mbed_assert_stub.c
+  stubs/mbed_critical_stub.c
   stubs/equeue_stub.c
   ../features/nanostack/coap-service/test/coap-service/unittest/stub/mbedtls_stub.c
   stubs/EventQueue_stub.cpp

UNITTESTS/features/netsocket/TLSSocket/unittest.cmake

@@ -21,6 +21,7 @@ set(unittest-test-sources
   features/netsocket/TLSSocket/test_TLSSocket.cpp
   stubs/Mutex_stub.cpp
   stubs/mbed_assert_stub.c
+  stubs/mbed_critical_stub.c
   stubs/equeue_stub.c
   ../features/nanostack/coap-service/test/coap-service/unittest/stub/mbedtls_stub.c
   stubs/EventQueue_stub.cpp

features/netsocket/InternetSocket.cpp

@@ -207,9 +207,9 @@ void InternetSocket::event()
 
 void InternetSocket::sigio(Callback<void()> callback)
 {
-    _lock.lock();
+    core_util_critical_section_enter();
     _callback = callback;
-    _lock.unlock();
+    core_util_critical_section_exit();
 }
 
 void InternetSocket::attach(Callback<void()> callback)