kernel: dynamic: reduce verbosity in degenerate case

k_thread_stack_free() is designed to be called with any pointer
value. We return -EINVAL when an attempt is made to free an
invalid stack pointer.

This change reduces the verbosity in the degenerate case, when
the pointer is not obtained via k_thread_stack_alloc(), but
otherwise does not affect functionality.

If debug log verbosity is not enabled, we save a few bytes in
.text / .rodata / .strtab.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>

kernel/dynamic.c

@@ -156,7 +156,7 @@ int z_impl_k_thread_stack_free(k_thread_stack_t *stack)
 		k_free(stack);
 #endif
 	} else {
-		LOG_ERR("Invalid stack %p", stack);
+		LOG_DBG("Invalid stack %p", stack);
 		return -EINVAL;
 	}