net: use napi_id_valid helper

In commit 6597e8d3 ("netdev-genl: Elide napi_id when not present"),
napi_id_valid function was added. Use the helper to refactor open-coded
checks in the source.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Stefano Jordhani <sjordhani@gmail.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk> # for iouring
Link: https://patch.msgid.link/20250214181801.931-1-sjordhani@gmail.com


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