Remove an invalid size check up from btrfs_shrink_dev().

The new size should not larger than the device->total_bytes as it was
already verified before coming to here(i.e. new_size < old_size).

Remove invalid check up for btrfs_shrink_dev().
Signed-off-by: NJie Liu <jeff.liu@oracle.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>