In order to avoid rewriting FLAC files when cleaning them, the padding
should be kept for all transitional steps, and removed only at the end.

In case the 'clean' command is run several times, it will result in no
rewrite at all, since the tags will be written over the padding of the
same size (no change).

This will reduce HDD access, and speed up the 'clean' command.

Signed-off-by: Cédric Marie <cedric@hjuvi.lautre.net>