linuxkpi: Move `ARRAY_SIZE` to <linux/array_size.h>

The DRM generic code started to import this header directly in Linux
6.12.x. Let's move the definition of `ARRAY_SIZE()` (the only thing that
<lin

linuxkpi: Move `ARRAY_SIZE` to <linux/array_size.h>

The DRM generic code started to import this header directly in Linux
6.12.x. Let's move the definition of `ARRAY_SIZE()` (the only thing that
<linux/array_size.h> defines) to this header.

Also, include <linux/array_size.h> from the same headers as Linux. This
includes <linux/kernel.h>, so the change should not break anything.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56570

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