netfilter: nf_tables: use rhashtable_walk_enter instead of rhashtable_walk_init

rhashtable_walk_init() is deprecated and rhashtable_walk_enter() can be
used instead. rhashtable_walk_init() is wrapper function of
rhashtable_walk_enter() so that logic is actually same.
But rhashtable_walk_enter() doesn't return error hence error path
code can be removed.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>