linux/ena: Add AF_XDP zero-copy multi-buffer support

[zz85]

Add multi-buffer (scatter-gather) support for AF_XDP zero-copy mode on kernels >= 6.6 that expose the xsk_buff_add_frag() API.

Tx path:

• Add a frame-atomic batch fast path (xsk_tx_peek_release_desc_batch) that guarantees complete frames are never truncated on the wire.
• Fall back to per-descriptor peek for shared-pool configurations.
• Track per-packet descriptor count (xsk_tx_nr_descs) for correct completion accounting.

Rx path:

• Assemble multi-descriptor packets into xdp_buff frag chains.
• Size XDP_PASS skbs for the full reassembled packet to prevent skb_shared_info corruption on __skb_put().
• Properly free frag chains on FORWARD/DROP/RECYCLE/alloc-failure to prevent double-free and use-after-free.

Advertise xdp_zc_max_segs clamped to min(tx_sgl, rx_sgl, MAX_SKB_FRAGS+1) so the stack never hands frames the datapath cannot fit.

Guarded by ENA_XSK_MB_SUPPORT (ENA_AF_XDP_SUPPORT && ENA_HAVE_XSK_MULTI_BUFF).

Issue #, if available:

Description of changes:

This challenge allows receiving and sending of Jumbo frames using AF_XDP + Zero copy with the ENA driver

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[davidarinzon]

Hi @zz85

Thank you for submitting this pull request, apologies for not replying sooner.
The feature is indeed on our agenda, we will take a look and analyze the pull request.