m68k: Fix virt_addr_valid() W=1 compiler warnings

If CONFIG_DEBUG_SG=y, and CONFIG_MMU=y:

    include/linux/scatterlist.h: In function ‘sg_set_buf’:
    arch/m68k/include/asm/page_mm.h:174:49: warning: ordered comparison of pointer with null pointer [-Wextra]
      174 | #define virt_addr_valid(kaddr) ((void *)(kaddr) >= (void *)PAGE_OFFSET && (void *)(kaddr) < high_memory)
	  |                                                 ^~

or CONFIG_MMU=n:

    include/linux/scatterlist.h: In function ‘sg_set_buf’:
    arch/m68k/include/asm/page_no.h:33:50: warning: ordered comparison of pointer with null pointer [-Wextra]
       33 | #define virt_addr_valid(kaddr) (((void *)(kaddr) >= (void *)PAGE_OFFSET) && \
	  |                                                  ^~

Fix this by doing the comparison in the "unsigned long" instead of the
"void *" domain.

Note that for now this is only seen when compiling btrfs, due to commit
e9aa7c2 ("btrfs: enable W=1 checks for btrfs"), but as people
are doing more W=1 compile testing, it will start to show up elsewhere,
too.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Link: https://lore.kernel.org/r/20210305084122.4118826-1-geert@linux-m68k.org

arch/m68k/include/asm/page_mm.h

@@ -171,7 +171,7 @@ static inline __attribute_const__ int __virt_to_node_shift(void)
 #include <asm-generic/memory_model.h>
 #endif
 
-#define virt_addr_valid(kaddr)	((void *)(kaddr) >= (void *)PAGE_OFFSET && (void *)(kaddr) < high_memory)
+#define virt_addr_valid(kaddr)	((unsigned long)(kaddr) >= PAGE_OFFSET && (unsigned long)(kaddr) < (unsigned long)high_memory)
 #define pfn_valid(pfn)		virt_addr_valid(pfn_to_virt(pfn))
 
 #endif /* __ASSEMBLY__ */

arch/m68k/include/asm/page_no.h

@@ -30,8 +30,8 @@ extern unsigned long memory_end;
 #define page_to_pfn(page)	virt_to_pfn(page_to_virt(page))
 #define pfn_valid(pfn)	        ((pfn) < max_mapnr)
 
-#define	virt_addr_valid(kaddr)	(((void *)(kaddr) >= (void *)PAGE_OFFSET) && \
-				((void *)(kaddr) < (void *)memory_end))
+#define	virt_addr_valid(kaddr)	(((unsigned long)(kaddr) >= PAGE_OFFSET) && \
+				((unsigned long)(kaddr) < memory_end))
 
 #endif /* __ASSEMBLY__ */