[IPV6]: Adjust inet6_exit() cleanup sequence against inet6_init()

This patch for adjust inet6_exit() to inverse sequence to inet6_init().

At ipv6_init, it first create proc_root/net/dev_snmp6 entry by call
ipv6_misc_proc_init(), then call addrconf_init() to create the corresponding
device entry at this directory, but at inet6_exit, ipv6_misc_proc_exit()
called first, then call addrconf_init().

Signed-off-by: Joe Jin <joe.jin@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>