e4defrag: output size per extent by -c option

e4defrag: output size per extent by -c option

e4defrag with -c option outputs "ratio" that means the levels of
fragmentation. However, it's difficult for users to understand, so we will
use size per extent instead of ratio.

Before:
# e4defrag -c /mnt/mp1/file
<File>                                         now/best          ratio
/mnt/mp1/file                                    6/1             0.00%

 Total/best extents                             6/1
 Fragmentation ratio                            0.00%
 Fragmentation score                            0.04
 [0-30 no problem: 31-55 a little bit fragmented: 55- needs defrag]
 This file(/mnt/mp1/file) does not need defragmentation.
 Done.

After:
# e4defrag -c /mnt/mp1/file
<File>                                         now/best       size/ext
/mnt/mp1/file                                    6/1          16666 KB

 Total/best extents                             6/1
 Average size per extent                        16666 KB
 Fragmentation score                            0
 [0-30 no problem: 31-55 a little bit fragmented: 56- needs defrag]
 This file (/mnt/mp1/file) does not need defragmentation.
 Done.

Signed-off-by: Kazuya Mio <k-mio@sx.jp.nec.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>