tuntap: dont use a private kmem_cache

tuntap: dont use a private kmem_cache

Commit 96442e42429 (tuntap: choose the txq based on rxq)
added a per tun_struct kmem_cache.

As soon as several tun_struct are used, we get an error
because two caches cannot have same name.

Use the default kmalloc()/kfree_rcu(), as it reduce code
size and doesn't have performance impact here.

Reported-by: Paul Moore <[email protected]>
Tested-by: Paul Moore <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Acked-by: Stephen Hemminger <[email protected]>
Acked-by: Jason Wang <[email protected]>
Signed-off-by: David S. Miller <[email protected]>