[NETNS][IPV6] mcast - fix compilation warning when procfs is not compiled in

When CONFIG_PROC_FS=no, the out_sock_create label is not used because
the code using it is disabled and that leads to a warning at compile
time.

This patch fix that by making a specific function to initialize proc
for igmp6, and remove the annoying CONFIG_PROC_FS sections in
init/exit function.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>