x86: early_ioremap_init(), use __fix_to_virt(), because we are sure it's safe

Tetsuo Handa reported this link bug:

 |  arch/x86/mm/built-in.o(.init.text+0x1831): In function `early_ioremap_init':
 |  : undefined reference to `__this_fixmap_does_not_exist'
 |  make: *** [.tmp_vmlinux1] Error 1

Commit:8827247ffcc9e880cbe4705655065cf011265157 used a variable (which
would be optimized to constant) as fix_to_virt()'s parameter.
It's depended on gcc's optimization and fails on old gcc. (Tetsuo used gcc 3.3)

We can use __fix_to_vir() instead, because we know it's safe and
don't need link time error reporting.

Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Cc: sfr@canb.auug.org.au
LKML-Reference: <49C9FFEA.7060908@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/mm/ioremap.c

@@ -516,7 +516,7 @@ void __init early_ioremap_init(void)
 		printk(KERN_INFO "early_ioremap_init()\n");
 
 	for (i = 0; i < FIX_BTMAPS_SLOTS; i++)
-		slot_virt[i] = fix_to_virt(FIX_BTMAP_BEGIN - NR_FIX_BTMAPS*i);
+		slot_virt[i] = __fix_to_virt(FIX_BTMAP_BEGIN - NR_FIX_BTMAPS*i);
 
 	pmd = early_ioremap_pmd(fix_to_virt(FIX_BTMAP_BEGIN));
 	memset(bm_pte, 0, sizeof(bm_pte));