x86/boot: use constant in head.S instead of hardcoded value

..to access multiboot.mem_lower data.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/boot/head.S

@@ -101,7 +101,7 @@ __start:
          */
         testb   $MBI_MEMLIMITS,(%ebx)
         jz      2f                  /* not available? BDA value will be fine */
-        mov     4(%ebx),%edx
+        mov     MB_mem_lower(%ebx),%edx
         cmp     $0x100,%edx         /* is the multiboot value too small? */
         jb      2f                  /* if so, do not use it */
         shl     $10-4,%edx

xen/arch/x86/x86_64/asm-offsets.c

@@ -165,4 +165,5 @@ void __dummy__(void)
 
     OFFSET(MB_flags, multiboot_info_t, flags);
     OFFSET(MB_cmdline, multiboot_info_t, cmdline);
+    OFFSET(MB_mem_lower, multiboot_info_t, mem_lower);
 }