Add very basic page pool use so that we can exercise
the netlink uAPI in a selftest.

Page pool gets created on open, destroyed on close.
But we control allocating of a single page thru debugfs.
This page may survive past the page pool itself so that
we can test orphaned page pools.

Link: https://lore.kernel.org/r/20240412141436.828666-2-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>