[PATCH 6.12.y 0/2] nft_byteorder: fix the uninitialised-stack read without removing multi-register support

[Abdifatah Suruur <suruurism@gmail.com>]

Pablo pointed out that the full upstream removal (bb061d3) cannot be
applied to stable trees: nftables versions before 1.1.0 can generate
len != size byteorder expressions, and the removal rejects them, so
existing rulesets may fail to load.  The removal is tagged
stable+noautosel for that reason.

This series restores the multi-register path in 6.12.y and fixes the
uninitialised-stack read with a targeted initialisation:

1/2 reverts the removal currently in 6.12.y.
2/2 zeroes the destination span marked initialised by the control
    plane, but only when source and destination registers differ, so
    in-place conversions (sreg == dreg) keep their value.

This does not reject any rule and keeps the old userspace-visible
behaviour.  It removes the dependency on CONFIG_INIT_STACK_ALL_ZERO
zeroing the register file.

Compile-tested by reverting the removal and applying 2/2 to the
pre-removal tree; runtime testing on 6.12.y is welcome.

Abdifatah Suruur (2):
  Revert "netfilter: nft_byteorder: remove multi-register support"
  netfilter: nft_byteorder: initialise the marked destination span

Pablo, this is the stable-specific alternative we discussed. If a single
folded patch is preferred, I can resend it that way.