gre: Count GRE packet drops

gre: Count GRE packet drops

GRE is silently dropping packets without updating statistics.

In case of drop, increment rx_dropped counter to provide visibility into
packet loss. For the case where no GRE protocol handler is registered,
use rx_nohandler.

Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Nimrod Oren <noren@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Link: https://patch.msgid.link/20260409090945.1542440-1-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>