samples/landlock: Fix incorrect free in populate_ruleset_net

samples/landlock: Fix incorrect free in populate_ruleset_net

Pointer env_port_name changes after strsep(). Memory allocated via
strdup() will not be freed if landlock_add_rule() returns non-zero value.

Fixes: 5e990dcef12e ("samples/landlock: Support TCP restrictions")
Signed-off-by: Ivanov Mikhail <ivanov.mikhail1@huawei-partners.com>
Reviewed-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Link: https://lore.kernel.org/r/20240326095625.3576164-1-ivanov.mikhail1@huawei-partners.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>