eth: fbnic: use netmem_ref where applicable

eth: fbnic: use netmem_ref where applicable

Use netmem_ref instead of struct page pointer in prep for
unreadable memory. fbnic has separate free buffer submission
queues for headers and for data. Refactor the helper which
returns page pointer for a submission buffer to take the
high level queue container, create a separate handler
for header and payload rings. This ties the "upcast" from
netmem to system page to use of sub0 which we know has
system pages.

Reviewed-by: Mina Almasry <almasrymina@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250901211214.1027927-5-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>