Rolf Neugebauer reported very long delays at netns dismantle.

Eric W. Biederman was kind enough to look at this problem
and noticed synchronize_net() occurring from netif_napi_del() that was
added in linux-4.5

Busy polling makes no sense for tunnels NAPI.
If busy poll is used for sessions over tunnels, the poller will need to
poll the physical device queue anyway.

netif_tx_napi_add() could be used here, but function name is misleading,
and renaming it is not stable material, so set NAPI_STATE_NO_BUSY_POLL
bit directly.

This will avoid inserting gro_cells napi structures in napi_hash[]
and avoid the problematic synchronize_net() (per possible cpu) that
Rolf reported.

Fixes: 93d05d4a ("net: provide generic busy polling to all NAPI drivers")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: NRolf Neugebauer <rolf.neugebauer@docker.com>
Reported-by: NEric W. Biederman <ebiederm@xmission.com>
Acked-by: NCong Wang <xiyou.wangcong@gmail.com>
Tested-by: NRolf Neugebauer <rolf.neugebauer@docker.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>