datapath: compat: Block upstream ip_tunnels functions.

datapath: compat: Block upstream ip_tunnels functions.

Since upstream and compat ip_tunnel structures are not same, we can not
use exported upstream functions.
Following patch blocks definitions which used ip_tunnel internal
structure. Function which do not depend on these structures are
allows by explicitly by defining it in the header files. e.g.
iptunnel_handle_offloads(), iptunnel_pull_header(). etc.

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