libata: accept late unlocking of HPA

On certain configurations, HPA isn't or can't be unlocked during
probing but it somehow ends up unlocked afterwards.  In the following
thread, the problem can be reliably reproduced after resuming from
STR.  The BIOS turns on HPA during boot but forgets to do it during
resume.

  http://thread.gmane.org/gmane.linux.kernel/858310

This patch updates libata revalidation such that it considers native
n_sectors.  If the device size has increased to match native
n_sectors, it's assumed that HPA has been unlocked involuntarily and
the device is recognized as the same one.  This should be fairly safe
while nicely working around the problem.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Christof Warlich <christof@warlich.name>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

include/linux/libata.h

@@ -589,6 +589,7 @@ struct ata_device {
 #endif
 	/* n_sector is CLEAR_BEGIN, read comment above CLEAR_BEGIN */
 	u64			n_sectors;	/* size of device, if ATA */
+	u64			n_native_sectors; /* native size, if ATA */
 	unsigned int		class;		/* ATA_DEV_xxx */
 	unsigned long		unpark_deadline;