PM/hibernate: fix "swap breaks after hibernation failures"

http://bugzilla.kernel.org/show_bug.cgi?id=12239

The image writing code dropped a reference to the current swap device.
This doesn't show up if the hibernation succeeds - because it doesn't
affect the image which gets resumed.  But it means multiple _failed_
hibernations end up freeing the swap device while it is still use!

swsusp_write() finds the block device for the swap file using swap_type_of().
It then uses blkdev_get() / blkdev_put() to open and close the block device.

Unfortunately, blkdev_get() assumes ownership of the inode of the block_device
passed to it.  So blkdev_put() calls iput() on the inode.  This is by design
and other callers expect this behaviour.  The fix is for swap_type_of() to take
a reference on the inode using bdget().
Signed-off-by: NAlan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>
Cc: Len Brown <lenb@kernel.org>
Cc: Greg KH <gregkh@suse.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -635,7 +635,7 @@ int swap_type_of(dev_t device, sector_t offset, struct block_device **bdev_p)
 
 		if (!bdev) {
 			if (bdev_p)
-				*bdev_p = sis->bdev;
+				*bdev_p = bdget(sis->bdev->bd_dev);
 
 			spin_unlock(&swap_lock);
 			return i;
@@ -647,7 +647,7 @@ int swap_type_of(dev_t device, sector_t offset, struct block_device **bdev_p)
 					struct swap_extent, list);
 			if (se->start_block == offset) {
 				if (bdev_p)
-					*bdev_p = sis->bdev;
+					*bdev_p = bdget(sis->bdev->bd_dev);
 
 				spin_unlock(&swap_lock);
 				bdput(bdev);