ip6_gre: correct the function name in ip6gre_tnl_addr_conflict() comment

The function name is wrong in ip6gre_tnl_addr_conflict() comment, which
use ip6_tnl_addr_conflict instead of ip6gre_tnl_addr_conflict.

Signed-off-by: Sun Lianwen <sunlw.fnst@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>