dpif-netlink: add GRE creation support

dpif-netlink: add GRE creation support

Creates GRE devices using rtnetlink and tunnel metadata. If the kernel does
not support tunnel metadata, it will return EEXIST because of the fallback
tunnel. However, on kernels between v3.10 and v3.12, it will not. So, we need to
verify the created tunnel has the tunnel metadata attribute.

This was tested on kernels 4.2.3, 4.3.6, 4.4.9, 4.5.5 and RHEL 3.10 based. All
of them worked with the system traffic test "datapath - ping over gre tunnel".
Also tested on a 3.10 based kernel without the fix for the existing fallback
tunnel.  That is, the kernel would not return EEXIST. Yet, the test works fine.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@redhat.com>