memory-failure: fix an error of mce_bad_pages statistics

memory-failure: fix an error of mce_bad_pages statistics

$ echo paddr > /sys/devices/system/memory/soft_offline_page to offline a
*free* page, the value of mce_bad_pages will be added, and the page is set
HWPoison flag, but it is still managed by page buddy alocator.

$ cat /proc/meminfo | grep HardwareCorrupted shows the value.

If we offline the same page, the value of mce_bad_pages will be added
*again*, this means the value is incorrect now.  Assume the page is still
free during this short time.

soft_offline_page()
	get_any_page()
			"else if (is_free_buddy_page(p))" branch return 0
						"goto done";
                                           "atomic_long_add(1, &mce_bad_pages);"

This patch:

Move poisoned page check at the beginning of the function in order to
fix the error.

Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Tested-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>