x86 setup: sizeof() is unsigned, unbreak comparisons

We use signed values for limit checking since the values can go
negative under certain circumstances.  However, sizeof() is unsigned
and forces the comparison to be unsigned, so move the comparison into
the heap_free() macros so we can ensure it is a signed comparison.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/arch/x86/boot/video.c b/arch/x86/boot/video.c
index e4ba897..ad9712f 100644
--- a/arch/x86/boot/video.c
+++ b/arch/x86/boot/video.c
@@ -371,7 +371,7 @@
 	saved.curx = boot_params.screen_info.orig_x;
 	saved.cury = boot_params.screen_info.orig_y;
 
-	if (heap_free() < saved.x*saved.y*sizeof(u16)+512)
+	if (!heap_free(saved.x*saved.y*sizeof(u16)+512))
 		return;		/* Not enough heap to save the screen */
 
 	saved.data = GET_HEAP(u16, saved.x*saved.y);