image_sender: final fix for copying not whole block device

There is an error, because df show's "Size" less then the whole fs
size.

And because of this, after I copied one /dev/sdX to another /dev/sdX I
have many many errors in dmesg:

[7326712.952640] EXT4-fs error (device sdd1): ext4_ext_find_extent:715:
inode #8773670: comm find: bad header/extent: invalid magic - magic
5555, entries 21845, max 21845(0), depth 21845(0)
(and others)
image_sender: copy a little more than we need (reserve, just in case)