netlink: specs: tc: use tc-gact instead of tc-gen as struct name

netlink: specs: tc: use tc-gact instead of tc-gen as struct name

There is a define in the uAPI header called tc_gen which expands
to the "generic" TC action fields. This helps other actions include
the base fields without having to deal with nested structs.

A couple of actions (sample, gact) do not define extra fields,
so the spec used a common tc-gen struct for both of them.
Unfortunately this struct does not exist in C. Let's use gact's
(generic act's) struct for basic actions.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250520161916.413298-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>