net: stub tcp_gro_complete if CONFIG_INET=n

A few networking drivers including bnx2x, bnxt, qede, and idpf call
tcp_gro_complete as part of offloading TCP GRO. The function is only
defined if CONFIG_INET is true, since its TCP specific and is meaningless
if the kernel lacks IP networking support.

The combination of trying to use the complex network drivers with
CONFIG_NET but not CONFIG_INET is rather unlikely in practice: most use
cases are going to need IP networking.

The tcp_gro_complete function just sets some data in the socket buffer for
use in processing the TCP packet in the event that the GRO was offloaded to
the device. If the kernel lacks TCP support, such setup will simply go
unused.

The bnx2x, bnxt, and qede drivers wrap their TCP offload support in
CONFIG_INET checks and skip handling on such kernels.

The idpf driver did not check CONFIG_INET and thus fails to link if the
kernel is configured  with CONFIG_NET=y, CONFIG_IDPF=(m|y), and
CONFIG_INET=n.

While checking CONFIG_INET does allow the driver to bypass significantly
more instructions in the event that we know TCP networking isn't supported,
the configuration is unlikely to be used widely.

Rather than require driver authors to care about this, stub the
tcp_gro_complete function when CONFIG_INET=n. This allows drivers to be
left as-is. It does mean the idpf driver will perform slightly more work
than strictly necessary when CONFIG_INET=n, since it will still execute
some of the skb setup in idpf_rx_rsc. However, that work would be performed
in the case where CONFIG_INET=y anyways.

I did not change the existing drivers, since they appear to wrap a
significant portion of code when CONFIG_INET=n. There is little benefit in
trashing these drivers just to unwrap and remove the CONFIG_INET check.

Using a stub for tcp_gro_complete is still beneficial, as it means future
drivers no longer need to worry about this case of CONFIG_NET=y and
CONFIG_INET=n, which should reduce noise from buildbots that check such a
configuration.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Link: https://lore.kernel.org/r/20231013185502.1473541-1-jacob.e.keller@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>