flow_offload: improve extack msg for user when adding invalid filter

Add extack message to return exact message to user when adding invalid
filter with conflict flags for TC action.

In previous implement we just return EINVAL which is confusing for user.

Signed-off-by: Baowen Zheng <baowen.zheng@corigine.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Link: https://lore.kernel.org/r/1646191769-17761-1-git-send-email-baowen.zheng@corigine.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>