netfilter: nf_tables: bogus EBUSY when deleting flowtable after flush (for 5.4)

3f0465a9 ("netfilter: nf_tables: dynamically allocate hooks per
net_device in flowtables") reworks flowtable support to allow for
dynamic allocation of hooks, which implicitly fixes the following
bogus EBUSY in transaction:

  delete flowtable
  add flowtable # same flowtable with same devices, it hits EBUSY

This patch does not exist in any tree, but it fixes this issue for
-stable Linux kernel 5.4

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>