netfilter: nf_tables: use dev->name directly

no need to store the name in separate area.

Furthermore, it uses kmalloc but not kfree and most accesses seem to treat
it as char[IFNAMSIZ] not char *.

Remove this and use dev->name instead.

In case event zeroed dev, just omit the name in the dump.

Fixes: d92191aa ("netfilter: nf_tables: cache device name in flowtable object")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>