net: openvswitch: add log message for error case

As requested by upstream OVS, added some error messages in the
validate_and_copy_dec_ttl function.

Includes a small cleanup, which removes an unnecessary parameter
from the dec_ttl_exception_handler() function.

Reported-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Link: https://lore.kernel.org/r/161054576573.26637.18396634650212670580.stgit@ebuild
Signed-off-by: Jakub Kicinski <kuba@kernel.org>