e2image: clean up progress reporting

e2image: clean up progress reporting

The old progress reporting code would crash on small file systems.
For example:

    cp /dev/null /tmp/foo.img
    mke2fs -t ext4 -F /tmp/foo.img 100
    e2image -o 0 -O 4096 -rap /tmp/foo.img

Fix this, and while we're at it, factor out the code to make it easier
to read and maintain.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>