Merge branch 'xdp-a-fistful-of-generic-changes-pt-i'

Alexander Lobakin says:

====================
xdp: a fistful of generic changes pt. I

XDP for idpf is currently 6 chapters:
* convert Rx to libeth;
* convert Tx and stats to libeth;
* generic XDP and XSk code changes (you are here);
* generic XDP and XSk code additions;
* actual XDP for idpf via new libeth_xdp;
* XSk for idpf (via ^).

Part III does the following:
* improve &xdp_buff_xsk cacheline placement;
* does some cleanups with marking read-only bpf_prog and xdp_buff
  arguments const for some generic functions;
* allows attaching already registered XDP memory model to RxQ info;
* makes system percpu page_pools valid XDP memory models;
* starts using netmems in the XDP core code (1 function);
* allows mixing pages from several page_pools within one XDP frame;
* optimizes &xdp_frame layout and removes no-more-used field.

Bullets 4-6 are the most important ones. All of them are prereqs to
libeth_xdp.
====================

Link: https://patch.msgid.link/20241203173733.3181246-1-aleksander.lobakin@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>