bpf: Add sockptr support for getsockopt

bpf: Add sockptr support for getsockopt

The whole network stack uses sockptr, and while it doesn't move to
something more modern, let's use sockptr in getsockptr BPF hooks, so, it
could be used by other callers.

The main motivation for this change is to use it in the io_uring
{g,s}etsockopt(), which will use a userspace pointer for *optval, but, a
kernel value for optlen.

Link: https://lore.kernel.org/all/ZSArfLaaGcfd8LH8@gmail.com/
Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20231016134750.1381153-2-leitao@debian.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>