net: stmmac: copy XDP metadata in AF_XDP zero-copy RX

stmmac_construct_skb_zc() reserves headroom for the XDP metadata and
records the length reported by the XDP program, but only copies the
packet data starting at xdp->data. The bytes in
[xdp->data_meta, xdp->data) are never copied into the new skb, so the
skb advertises metadata that is not there: after eth_type_trans()
anchors mac_header, data_meta points at uninitialized headroom instead
of what the program wrote.

Copy the metadata together with the packet data starting at
xdp->data_meta, leaving skb->data at the start of the frame and the
metadata immediately before it, and record its length. This keeps
data_meta valid for the stack and TC BPF after eth_type_trans() anchors
mac_header.

Fixes: bba2556efad6 ("net: stmmac: Enable RX via AF_XDP zero-copy")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@oss.qualcomm.com>
Link: https://patch.msgid.link/20261027-stmmac-xdp-fixes-v2-4-10eae8a093a8@oss.qualcomm.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>