[NETNS]: Re-export init_net via EXPORT_SYMBOL.

init_net is used added as a parameter to a lot of old API calls, f.e.
ip_dev_find. These calls were exported as EXPORT_SYMBOL. So, export init_net
as EXPORT_SYMBOL to keep networking API consistent.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>