linuxkpi: Add `kmemdup_array()`

[dumbbell]

Summary

It is similar to kmemdup() but takes a number of elements to duplicate and their size.

The i915 DRM driver started to use it in Linux 6.12.

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/string.h

108

I believe there is no need for a local variable but that's just noise.

I would add size_mul() to limit overflows like I did in f1834d03af617a93993056bb902082452673f89f .

{
       return (kmemdup(src, size_mul(count, element_size), gfp));
}

[dumbbell]

Address comment from @bz