inet: frags: delay fqdir_free_fn()

fqdir_free_fn() is using very expensive rcu_barrier()

When one netns is dismantled, we often call fqdir_exit()
multiple times, typically lauching fqdir_free_fn() twice.

Delaying by one second fqdir_free_fn() helps to reduce
the number of rcu_barrier() calls, and lock contention
on rcu_state.barrier_mutex.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>