linuxkpi: Add Linux 6.12 variant of `kvrealloc()`

[dumbbell]

Summary

In Linux 6.12, the API changed to be closer to krealloc():

• The function does not take the old size anymore
• The function becomes a wrapper around krealloc() with a fallback mechanism.

This is part of the update of DRM drivers to Linux 6.12.

Sponsored by: The FreeBSD Foundation

[bz]

sys/compat/linuxkpi/common/include/linux/slab.h

252

Again, I believe this should be the new default and the old version should require LINUXKPI_VERSION.

270

So if ptr is NULL we are in bad shape anyway also in the old version.

I believe what we could do to simplify this is:

size_t oldsize = ksize(ptr);

_lkpi_kvrealloc( ... ) which is the old versions and then the old version is a plain wrapper around the __lkpi_kvrealloc() one used by both. That way it is a lot less code duplication. And is we want we can add the ptr != NULL check there too if really needed.