journald: whenever we rotate a file, btrfs defrag it

journald: whenever we rotate a file, btrfs defrag it

Our write pattern is quite awful for CoW file systems (btrfs...), as we
keep updating file parts in the beginning of the file. This results in
fragmented journal files. Hence: when rotating files, defragment them,
since at that point we know that no further write accesses will be made.