netfilter: nf_tables: Keep deleted flowtable hooks until after RCU

Documentation of list_del_rcu() warns callers to not immediately free
the deleted list item. While it seems not necessary to use the
RCU-variant of list_del() here in the first place, doing so seems to
require calling kfree_rcu() on the deleted item as well.

Fixes: 3f0465a9 ("netfilter: nf_tables: dynamically allocate hooks per net_device in flowtables")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>