swapon: align --summary fields

This commit makes  partitions and files to have remaining three columns
aligned.  Below print out demonstrates earlier misalignment.

$ swapon -s
Filename                                Type            Size    Used    Priority
/dev/sda2                               partition       4194300 0       3
/home/src/util-linux/newswap            file    496     0       -1

Signed-off-by: Sami Kerola <kerolasa@iki.fi>