tunnel: Mark GRE64 tunnel protocol deprecated.

tunnel: Mark GRE64 tunnel protocol deprecated.

GRE64 was introduced to extend GRE key from 32-bit to 64-bit using
key and sequence number field. But GRE64 is not standard
protocol. There are not many users of this protocol. Therefore we
have decided to deprecate it. Since GRE64 is not in upstream
OVS kernel module removing it will bring upstream and out-of-tree
module closer.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>