Replaces snapshot-of-tx_tail comparison in the watchdog with a
tx_stall_tail_moved flag set/cleared under tx_ptr_lock. Avoids
the index-aliasing false-positive Phil Elwell flagged on the
raspberrypi/linux PR review.
Anchored against the rpi-6.18.y vendor fork so it applies cleanly
on top of patches 0001..0003. See talos-kernel-patches/0004-*.patch
for the mainline-anchored variant intended for netdev v2.
