lib/dpif-netlink: Fix miscompare of gre ports

lib/dpif-netlink: Fix miscompare of gre ports

In netdev_to_ovs_vport_type() it checks for netdev types matching
"gre" with a strstr().  This makes it match ip6gre as well and return
OVS_VPORT_TYPE_GRE, which is clearly wrong.

Move the usage of strstr() *after* all the exact matches with strcmp()
to avoid the problem permanently because when I added the ip6gre
type I ran into a very difficult to detect bug.

Cc: Ben Pfaff <blp@ovn.org>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: William Tu <u9012063@gmail.com>