[SRU I/F 0/1] Fix xfrm interface no longer changeable

[Stefan Bader]

BugLink: https://bugs.launchpad.net/bugs/1968591

[Impact]

An xfrm interface cannot be changed any more since Ubuntu-hwe-5.4-5.4.0-105
(also Impish, see additional notes). In fact, the regression has been
introduced by this backport:
https://kernel.ubuntu.com/git/ubuntu/ubuntu-focal.git/commit/?id=13a02539b135

It has been fixed upstream by this commit:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6d0d95a1c2b0

[Test Case]

root at dut-vm:~# ip link add foo type xfrm if_id 1234 dev lo
root at dut-vm:~# ip link change foo type xfrm if_id 1234 dev ntfp1
Error: if_id must be non zero.
root at dut-vm:~# uname -a
Linux dut-vm 5.4.0-107-generic #121~18.04.1-Ubuntu SMP Thu Mar 24 17:21:33 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
root at dut-vm:~#

[Regression Potential]

The patch is trivial, the potential regressions are low.

Additional notes:

The change which introduces the issue also exists in Impish(5.13.0-40.45) and
would need the same fix. For Focal the fix would be included in upstream stable
v5.4.183 but that would not make it into next cycle.