selftests/landlock: Fix error message

[ Upstream commit 2107c35128ad751b201eb92fe91443450d9e5c37 ]

The global variable errno may not be set in test_execute().  Do not use
it in related error message.

Cc: Günther Noack <gnoack@google.com>
Fixes: e1199815 ("selftests/landlock: Add user space tests")
Link: https://lore.kernel.org/r/20250108154338.1129069-21-mic@digikod.net


Signed-off-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>