selftests: ncdevmem: Unify error handling

There is a bunch of places where error() calls look out of place.
Use the same error(1, errno, ...) pattern everywhere.

Reviewed-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20241107181211.3934153-4-sdf@fomichev.me


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