arch/x86/kernel/e820.c: quiet sparse noise about plain integer as NULL pointer

The last parameter to sort() is a pointer to the function used
to swap items.  This parameter should be NULL, not 0, when not
used.  This quiets the following sparse warning:

 warning: Using plain integer as NULL pointer

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: hartleys@visionengravers.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>