linuxkpi: Define `ZERO_OR_NULL_PTR()` in <linux/slab.h>

linuxkpi: Define `ZERO_OR_NULL_PTR()` in <linux/slab.h>

On Linux, the `kmalloc()` family of functions returns a special value if
the size of the allocation is zero. This macro verifies if the pointer
is NULL (the allocation failed) or the size is 0 (the allocation was not
performed AFAIU). This special value can be passed to `kfree()`.

On FreeBSD, our `malloc(9)` functions don't return a special value for
0-size allocations. Therefore we can simply compare the result against
NULL.

Reviewed by:	manu
Approved by:	manu
Differential Revision:	https://reviews.freebsd.org/D37367