gro_cells: avoid using synchronize_rcu() in gro_cells_destroy()

Another thing making netns dismantles potentially very slow is located
in gro_cells_destroy(),
whenever cleanup_net() has to remove a device using gro_cells framework.

RTNL is not held at this stage, so synchronize_net()
is calling synchronize_rcu():

netdev_run_todo()
 ip_tunnel_dev_free()
  gro_cells_destroy()
   synchronize_net()
    synchronize_rcu() // Ouch.

This patch uses call_rcu(), and gave me a 25x performance improvement
in my tests.

cleanup_net() is no longer blocked ~10 ms per synchronize_rcu()
call.

In the case we could not allocate the memory needed to queue the
deferred free, use synchronize_rcu_expedited()

v2: made percpu_free_defer_callback() static

Signed-off-by: Eric Dumazet <[email protected]>
Acked-by: Paolo Abeni <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

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