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%[email protected]/

Signed-off-by: Marek Behún <[email protected]>
Signed-off-by: Christian Lamparter <[email protected]> (squashed)