net: macb: check TX ring before modifying skb

macb_pad_and_fcs() replaces or extends the skb before the ring space
check. On NETDEV_TX_BUSY the stack requeues an skb that is already freed
or grown.

Check the ring first, using the padded length for the descriptor count.
Nonlinear skbs always take the copy path so the count can assume a
linear skb.

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-v1-1-a80641479041@tipi-net.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>