ipv6_tunnel: Rate limit warning messages

commit 6cee105e7f2ced596373951d9ea08dacc3883c68 upstream.

The warning messages can be invoked from the data path for every packet
transmitted through an ip6gre netdev, leading to high CPU utilization.

Fix that by rate limiting the messages.

Fixes: 09c6bbf0 ("[IPV6]: Do mandatory IPv6 tunnel endpoint checks in realtime")
Reported-by: Maksym Yaremchuk <maksymy@nvidia.com>
Tested-by: Maksym Yaremchuk <maksymy@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>