ipv6: Fix the link time qualifier of 'ping_v6_proc_exit_net()'

The '.exit' functions from 'pernet_operations' structure should be marked
as __net_exit, not __net_init.

Fixes: d862e546

 ("net: ipv6: Implement /proc/net/icmp6.")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>