netfilter: nf_tables: close race between netns exit and rmmod

If net namespace is exiting while nf_tables module is being removed
we can oops:

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000040
 IP: nf_tables_flowtable_event+0x43/0xf0 [nf_tables]
 PGD 0 P4D 0
 Oops: 0000 [#1] SMP PTI
 Modules linked in: nf_tables(-) nfnetlink [..]
  unregister_netdevice_notifier+0xdd/0x130
  nf_tables_module_exit+0x24/0x3a [nf_tables]
  SyS_delete_module+0x1c5/0x240
  do_syscall_64+0x74/0x190

Avoid this by attempting to take reference on the net namespace from
the notifiers.  If it fails the namespace is exiting already, and nft
core is taking care of cleanup work.

We also need to make sure the netdev hook type gets removed
before netns ops removal, else notifier might be invoked with device
event for a netns where net->nft was never initialised (because
pernet ops was removed beforehand).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>