gre6: add collect metadata support

gre6: add collect metadata support

The patch adds 'external' option to support collect metadata
gre6 tunnel.  The 'external' keyword is already used to set the
device into collect metadata mode such as vxlan, geneve, ipip,
etc.  This patch extends support for ipv6 gre and gretap.
Example of L3 and L2 gre device:
bash:~# ip link add dev ip6gre123 type ip6gre external
bash:~# ip link add dev ip6gretap123 type ip6gretap external

Signed-off-by: William Tu <u9012063@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>