verify: rename corrupted chunks with .bad extension

verify: rename corrupted chunks with .bad extension

This ensures that following backups will always upload the chunk,
thereby replacing it with a correct version again.

Format for renaming is <digest>.<counter>.bad where <counter> is used if
a chunk is found to be bad again before a GC cleans it up.

Care has been taken to deliberately only rename a chunk in conditions
where it is guaranteed to be an error in the chunk itself. Otherwise a
broken index file could lead to an unwanted mass-rename of chunks.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>