Btrfs: When shrinking, only update disk size on success

Previously, we updated a device's size prior to attempting a shrink
operation.  This patch moves the device resizing logic to only happen if
the shrink completes successfully.  In the process, it introduces a new
field to btrfs_device -- disk_total_bytes -- to track the on-disk size.

Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/volumes.h

@@ -61,6 +61,9 @@ struct btrfs_device {
 	/* size of the device */
 	u64 total_bytes;
 
+	/* size of the disk */
+	u64 disk_total_bytes;
+
 	/* bytes used */
 	u64 bytes_used;