linuxkpi: Add `mem_is_zero()`

[dumbbell]

Summary

The DRM generic code and the i915 DRM driver started to replace the use of memchr_inv() by a mem_is_zero() in Linux 6.12. We reuse the previous use of memchr_inv() to implement mem_is_zero().

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

206

This could be written as
return (!memchr_inv(start, 0, length));
but as per style probably should be
return (memchr_inv(start, 0, length) == NULL);

[dumbbell]

Address comment from @bz

Sorry, that’s me who dislike putting calls or complex statements directly in the return :-) I should have followed the adopted style.