selftests: drv-net: improve the use of ksft helpers in XSK queue test

Avoid exceptions when xsk attr is not present, and add a proper ksft
helper for "not in" condition.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Tested-by: Kurt Kanzenbach <kurt@linutronix.de>
Tested-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250219234956.520599-7-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>