nft: fix interface wildcard matching

nft: fix interface wildcard matching

In (73ea1cc nft: convert rule into a command state structure), the
interface wildcard matching got broken. The previous handling was
flawed by the use of ifnametoindex in scenario where the interface
may vanished after a rule was added.

This approach relies on the trailing '\0' to identify if this is
an exact or wildcard matching, based on discussion with Florian.

Based on initial patch from Anand Raj Manickam.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>