ip_tunnel: Add sanity checks to ip_tunnel_encap_add_ops()

The symbols are exported and could be used by external modules.

Fixes: a8c5f9 ("ip_tunnel: Ops registration for secondary encap (fou, gue)")
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>