net/mlx5e: XDP, Use page fragments for linear data in multibuf-mode

net/mlx5e: XDP, Use page fragments for linear data in multibuf-mode

Currently in XDP multi-buffer mode for striding rq a whole page is
allocated for the linear part of the XDP buffer. This is wasteful,
especially on systems with larger page sizes.

This change splits the page into fixed sized fragments. The page is
replenished when the maximum number of allowed fragments is reached.
When a fragment is not used, it will be simply recycled on next packet.
This is great for XDP_DROP as the fragment can be recycled for the next
packet. In the most extreme case (XDP_DROP everything), there will be 0
fragments used => only one linear page allocation for the lifetime of
the XDP program.

The previous page_pool size increase was too conservative (doubling the
size) and now there are much fewer allocations (1/8 for a 4K page). So
drop the page_pool size extension altogether when the linear side page
is used.

This small improvement is at most visible for XDP_DROP tests with small
64B packets and a large enough MTU for Striding RQ to be in non-linear
mode:
+----------------------------------------------------------------------+
| System               | MTU  | baseline   | this change | improvement |
|----------------------+------+------------+-------------+-------------|
| 4K page x86_64 [1]   | 9000 | 26.30 Mpps | 30.45 Mpps  | 15.80 %     |
| 64K page aarch64 [2] | 9000 | 15.27 Mpps | 20.10 Mpps  | 31.62 %     |
+----------------------------------------------------------------------+

[1] Intel Xeon Platinum 8580
[2] ARM Neoverse-N1

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Carolina Jubran <cjubran@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20260403090927.139042-6-tariqt@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>