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

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

@@ -90,6 +90,8 @@ struct linux_kmem_cache;
 /* drm-kmod 5.4 compat */
 #define kfree_async(ptr)	kfree(ptr);
 
+#define ZERO_OR_NULL_PTR(x)	((x) == NULL)
+
 static inline gfp_t
 linux_check_m_flags(gfp_t flags)
 {