selftests/net: Be consistent in kconfig checks

selftests/net: Be consistent in kconfig checks

Most of the functions in tcp-ao lib/ return negative errno or -1 in case
of a failure. That creates inconsistencies in lib/kconfig, which saves
what was the error code. As well as the uninitialized kconfig value is
-1, which also may be the result of a check.

Define KCONFIG_UNKNOWN and save negative return code, rather than
libc-style errno.

Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20240823-tcp-ao-selftests-upd-6-12-v4-3-05623636fe8c@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>