eth: fbnic: move page pool pointer from NAPI to the ring struct

eth: fbnic: move page pool pointer from NAPI to the ring struct

In preparation for memory providers we need a closer association
between queues and page pools. We used to have a page pool at the
NAPI level to serve all associated queues but with MP the queues
under a NAPI may no longer be created equal.

The "ring" structure in fbnic is a descriptor ring. We have separate
"rings" for payload and header pages ("to device"), as well as a ring
for completions ("from device"). Technically we only need the page
pool pointers in the "to device" rings, so adding the pointer to
the ring struct is a bit wasteful. But it makes passing the structures
around much easier.

For now both "to device" rings store a pointer to the same
page pool. Using more than one queue per NAPI is extremely rare
so don't bother trying to share a single page pool between queues.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250901211214.1027927-2-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>