Make multi-part MAC operations thread-safe

Within setup we create a copy of the key and put it in the operation field.
After setup, we only ever use the new copy - and do not interact with any key slots.
Therefore we need only register as a reader of the key during setup,
then unregister after we stop accessing the key.

Simultaneous API calls on the same operation object are not thread-safe.

Signed-off-by: Ryan Everett <ryan.everett@arm.com>

library/psa_crypto.c

@@ -2565,7 +2565,7 @@ exit:
         psa_mac_abort(operation);
     }
 
-    unlock_status = psa_unregister_read(slot);
+    unlock_status = psa_unregister_read_under_mutex(slot);
 
     return (status == PSA_SUCCESS) ? unlock_status : status;
 }