gtp: update rx_length_errors for abnormally short packets

Based on work by Pravin Shelar.

Update appropriate stats when packet transmission isn't possible.

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>