Merge branch 'net-use-skb_share_check-in-loopback-selftest-hooks'

Nicolai Buchwitz says:

====================
net: use skb_share_check() in loopback selftest hooks

The loopback selftest hooks can get a shared skb when another handler
is bound to the device. Linearizing a non-linear frame then hits
BUG_ON(skb_shared()) in pskb_expand_head().

Stumbled over this on RPi CM4 (bcmgenet, with jumbo series applied +
WIP selftest patches) with a packet socket open at the same time as
running ethtool -t.

stmmac and mlx5 seem to have the same pattern, xgbe already handles
shared skbs. Patches 2 and 3 are compile tested only.

As the impact is pretty limited and doesn't seem to bother anyone in
the wild, I've chosen net-next, but kept the Fixes tags. That matches
what Paolo suggested during the LPC track.
====================

Link: https://patch.msgid.link/20261007-nb-net-selftest-shared-net-v1-0-458369bfa1ba@tipi-net.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>