mm: Fix virt_to_page() warning

virt_to_page() is typically implemented as a macro containing a cast so
that it will accept both pointers and unsigned long without causing a
warning.

But MIPS virt_to_page() uses virt_to_phys which is a function so passing
an unsigned long will cause a warning:

    CC      mm/page_alloc.o
  mm/page_alloc.c: In function ‘free_reserved_area’:
  mm/page_alloc.c:5161:3: warning: passing argument 1 of ‘virt_to_phys’ makes pointer from integer without a cast [enabled by default]
  arch/mips/include/asm/io.h:119:100: note: expected ‘const volatile void *’ but argument is of type ‘long unsigned int’

All others users of virt_to_page() in mm/ are passing a void *.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Reported-by: Eunbong Song <eunb.song@samsung.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-mips@linux-mips.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -5158,7 +5158,7 @@ unsigned long free_reserved_area(unsigned long start, unsigned long end,
 	for (pages = 0; pos < end; pos += PAGE_SIZE, pages++) {
 		if (poison)
 			memset((void *)pos, poison, PAGE_SIZE);
-		free_reserved_page(virt_to_page(pos));
+		free_reserved_page(virt_to_page((void *)pos));
 	}
 
 	if (pages && s)