drbd: Remove the open-coded page pool

drbd: Remove the open-coded page pool

If the network stack keeps a reference for too long, DRBD keeps
references on a higher number of pages as a consequence.

Fix all that by no longer relying on page reference counts dropping to
an expected value. Instead, DRBD gives up its reference and lets the
system handle everything else. While at it, remove the open-coded
custom page pool mechanism and use the page_pool included in the
kernel.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Tested-by: Eric Hagberg <ehagberg@janestreet.com>
Link: https://lore.kernel.org/r/20250605103852.23029-1-christoph.boehmwalder@linbit.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>