linuxkpi: add padding to struct pci_driver

Add 32 or 64 bytes of padding to struct pci_driver at the end in the
_spare field like we should have done when we branched stable/13, but
neglected to do so since we didn't properly anticipate the need.

We cannot safely use these spare fields until after 13.0 EOL since
drivers compiled on 13.0 won't have that space reserved and we'll step
on something else using them. This isn't 100% KBI compatible through the
13.x release branch, but is compatible enough so that drm packages built
on the oldest supported release will work on the latest stable/13 and
any newer releases. It's not ideal, but makes the best of a bad
situation and is a pragmatic approach that belatedly builds in some
future proofing.

Direct commit to stable/13 because this is not relevant to main in this
exact form.

Approved by: re@ (gjb)
Sponsored by: Netflix
Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D34754

(cherry picked from commit 86d5c66106610f083deda03ce0c661a51b2dd723)