ip_tunnel: remove __net_init/exit from exported functions

ip_tunnel: remove __net_init/exit from exported functions

If CONFIG_NET_NS is not set then __net_init is the same as __init and
__net_exit is the same as __exit. These functions will be removed from
memory after the module loads or is removed. Functions that are exported
for use by other functions should never be labeled for removal.

Bug introduced by commit e7ccb3806ba4bf49634
("GRE: Refactor GRE tunneling code.")

Reported-by: Steinar H. Gunderson <sgunderson@bigfoot.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>