gtp: include role in link info

Querying link info for the GTP interface doesn't reveal in which "role" the
device is set to operate.  Include this information in the info query
result.

Signed-off-by: Jonas Bonn <jonas@norrbonn.se>
Acked-by: Harald Welte <laforge@gnumonks.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>