net: macb: copy shared skbs before appending the FCS

macb_pad_and_fcs() appends the FCS in place when the skb has tailroom.
A shared skb, as pktgen sends in clone_skb mode, grows by one FCS per
transmit. BQL then completes more bytes than were queued and
dql_completed() hits its BUG_ON.

On a Raspberry Pi CM5 (RP1 GEM) pktgen with clone_skb 1000 burst 32 at
60 bytes kills the box within seconds.

Copy shared skbs before appending the FCS. Clearing IFF_TX_SKB_SHARING
would also fix it but makes pktgen refuse clone_skb on macb.

Fixes: 653e92a9175e ("net: macb: add support for padding and fcs computation")
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20261006-nb-macb-shared-skb-net-v1-2-a80641479041@tipi-net.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>