netfilter: nf_tables: bogus EBUSY when deleting flowtable after flush

The deletion of a flowtable after a flush in the same transaction
results in EBUSY. This patch adds an activation and deactivation of
flowtables in order to update the _use_ counter.

Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>