bpf: udp: Make sure iter->batch always contains a full bucket snapshot

bpf: udp: Make sure iter->batch always contains a full bucket snapshot

Require that iter->batch always contains a full bucket snapshot. This
invariant is important to avoid skipping or repeating sockets during
iteration when combined with the next few patches. Before, there were
two cases where a call to bpf_iter_udp_batch may only capture part of a
bucket:

1. When bpf_iter_udp_realloc_batch() returns -ENOMEM [1].
2. When more sockets are added to the bucket while calling
   bpf_iter_udp_realloc_batch(), making the updated batch size
   insufficient [2].

In cases where the batch size only covers part of a bucket, it is
possible to forget which sockets were already visited, especially if we
have to process a bucket in more than two batches. This forces us to
choose between repeating or skipping sockets, so don't allow this:

1. Stop iteration and propagate -ENOMEM up to userspace if reallocation
   fails instead of continuing with a partial batch.
2. Try bpf_iter_udp_realloc_batch() with GFP_USER just as before, but if
   we still aren't able to capture the full bucket, call
   bpf_iter_udp_realloc_batch() again while holding the bucket lock to
   guarantee the bucket does not change. On the second attempt use
   GFP_NOWAIT since we hold onto the spin lock.

Introduce the udp_portaddr_for_each_entry_from macro and use it instead
of udp_portaddr_for_each_entry to make it possible to continue iteration
from an arbitrary socket. This is required for this patch in the
GFP_NOWAIT case to allow us to fill the rest of a batch starting from
the middle of a bucket and the later patch which skips sockets that were
already seen.

Testing all scenarios directly is a bit difficult, but I did some manual
testing to exercise the code paths where GFP_NOWAIT is used and where
ERR_PTR(err) is returned. I used the realloc test case included later
in this series to trigger a scenario where a realloc happens inside
bpf_iter_udp_batch and made a small code tweak to force the first
realloc attempt to allocate a too-small batch, thus requiring
another attempt with GFP_NOWAIT. Some printks showed both reallocs with
the tests passing:

Apr 25 23:16:24 crow kernel: go again GFP_USER
Apr 25 23:16:24 crow kernel: go again GFP_NOWAIT

With this setup, I also forced each of the bpf_iter_udp_realloc_batch
calls to return -ENOMEM to ensure that iteration ends and that the
read() in userspace fails.

[1]: https://lore.kernel.org/bpf/CABi4-ogUtMrH8-NVB6W8Xg_F_KDLq=yy-yu-tKr2udXE2Mu1Lg@mail.gmail.com/
[2]: https://lore.kernel.org/bpf/7ed28273-a716-4638-912d-f86f965e54bb@linux.dev/

Signed-off-by: Jordan Rife <jordan@jrife.io>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>