e2image: only skip writing zero blocks when writing to a new file

e2image: only skip writing zero blocks when writing to a new file

The e2image progam was originally intended to create image files.
However, some people have started using e2image to copy a file system
from one block device to another, since it is more efficient than
using dd because it only copies the blocks which are in use.  If we
are doing this, however, we must not skip writing blocks which are all
zero in the source device, since they may not be zero in the
destination device.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Phillip Susi <psusi@ubuntu.com>