page_pool: expose max page pool ring size

Expose this as a constant so we can reuse it in drivers.

cos-patch: bug
Change-Id: I656dc0392d9b2e2f9727e52bcacb9bbff3287a58
Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/116947
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: Kevin Berry <kpberry@google.com>
Main-Branch-Verified: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/118005