[x86 setup] Correct the SMAP check for INT 0x15, AX=0xe820

The e820 probe code was checking %edx, not %eax, for the SMAP
signature on return.  This worked on *almost* all systems, since %edx
still contained SMAP from the call on entry, but on a handful of
systems it failed -- plus, we would have missed real mismatches.

The error output is "=d" to make sure gcc knows %edx is clobbered
here.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/i386/boot/memory.c

@@ -28,11 +28,14 @@ static int detect_memory_e820(void)
 
 	do {
 		size = sizeof(struct e820entry);
-		id = SMAP;
+
+		/* Important: %edx is clobbered by some BIOSes,
+		   so it must be either used for the error output
+		   or explicitly marked clobbered. */
 		asm("int $0x15; setc %0"
-		    : "=am" (err), "+b" (next), "+d" (id), "+c" (size),
+		    : "=d" (err), "+b" (next), "=a" (id), "+c" (size),
 		      "=m" (*desc)
-		    : "D" (desc), "a" (0xe820));
+		    : "D" (desc), "d" (SMAP), "a" (0xe820));
 
 		/* Some BIOSes stop returning SMAP in the middle of
 		   the search loop.  We don't know exactly how the BIOS