ANDROID: Release a KABI field in struct net_device for backports

There are 8 ANDROID_KABI_RESERVE() fields in struct net_device.
We are now 3 years into the 6 year lifetime for android13-5.15 and
none of those 8 padding fields have been used. At this point it is
reasonable to allow 1 of those fields to be used for non-LTS backports.

Mark one of the fields as ANDROID_KABI_BACKPORT_OK()

Bug: 391829285
Bug: 392673958
Change-Id: Idfd9b46cb728e5a5942cb818a6f553c0853a5167
Signed-off-by: Todd Kjos <tkjos@google.com>