Make restartable signature operations thread-safe

We copy the key from a slot to the operation object in _start.
_complete and _abort do not access any key slots, instead using
the local copy.

Concurrently using the same operation object is not thread-safe.

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

library/psa_crypto.c

@@ -3371,7 +3371,7 @@ exit:
         psa_sign_hash_abort_internal(operation);
     }
 
-    unlock_status = psa_unregister_read(slot);
+    unlock_status = psa_unregister_read_under_mutex(slot);
 
     if (unlock_status != PSA_SUCCESS) {
         operation->error_occurred = 1;