ip_tunnels: convert the mode field of ip_tunnel_info to flags

The mode field holds a single bit of information only (whether the
ip_tunnel_info struct is for rx or tx). Change the mode field to bit flags.
This allows more mode flags to be added.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>