selftests/landlock: NULL-terminate unix pathname addresses

selftests/landlock: NULL-terminate unix pathname addresses

The size of Unix pathname addresses is computed in selftests using
offsetof(struct sockaddr_un, sun_path) + strlen(xxx). It should have
been that +1, which makes addresses passed to the libc and kernel
non-NULL-terminated. unix_mkname_bsd() fixes that in Linux so there is
no harm, but just using sizeof(the address struct) should improve
readability.

Signed-off-by: Matthieu Buffet <matthieu@buffet.re>
Reviewed-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20251202215141.689986-1-matthieu@buffet.re
Signed-off-by: Mickaël Salaün <mic@digikod.net>