[XFS] fix use after free with external logs or real-time devices

SGI-PV: 983806

SGI-Modid: xfs-linux-melb:xfs-kern:31666a
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>

fs/xfs/linux-2.6/xfs_super.c

@@ -792,12 +792,14 @@ xfs_close_devices(
 	struct xfs_mount	*mp)
 {
 	if (mp->m_logdev_targp && mp->m_logdev_targp != mp->m_ddev_targp) {
+		struct block_device *logdev = mp->m_logdev_targp->bt_bdev;
 		xfs_free_buftarg(mp->m_logdev_targp);
-		xfs_blkdev_put(mp->m_logdev_targp->bt_bdev);
+		xfs_blkdev_put(logdev);
 	}
 	if (mp->m_rtdev_targp) {
+		struct block_device *rtdev = mp->m_rtdev_targp->bt_bdev;
 		xfs_free_buftarg(mp->m_rtdev_targp);
-		xfs_blkdev_put(mp->m_rtdev_targp->bt_bdev);
+		xfs_blkdev_put(rtdev);
 	}
 	xfs_free_buftarg(mp->m_ddev_targp);
 }