vxlan: Change dpif_backer->tnl backer to a "struct simap"

Move dpif_backer->tnl_backers from a "struct sset" to a
"struct simap". Store odp_port in the new map.  This will make it easier to
access the odp_port for future patches.

Signed-off-by: Kyle Mestery <kmestery@cisco.com>
Acked-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>