kernel: Backport mvneta crash fix to 5.15

kernel: Backport mvneta crash fix to 5.15

Backport Russell King's series [1]
  net: mvneta: reduce size of TSO header allocation
to pending-5.15 to fix random crashes on Turris Omnia.

This also backports two patches that are dependencies to this series:
  net: mvneta: Delete unused variable
  net: mvneta: fix potential double-frees in mvneta_txq_sw_deinit()

[1] https://lore.kernel.org/netdev/ZCsbJ4nG+So%2Fn9qY@shell.armlinux.org.uk/

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com> (squashed)
(cherry picked from commit 7b31c2e9ed4da7bfeecbd393c17c249eca870717)