netdev-genl: Dump gro_flush_timeout

Support dumping gro_flush_timeout for a NAPI ID.

Signed-off-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241011184527.16393-5-jdamato@fastly.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>