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>