journal: Fix disabling NO_COW on btrfs filesystems

journal: Fix disabling NO_COW on btrfs filesystems

Disabling NOCOW when data has been written to a file doesn't work.
Instead, when we're done writing to a journal file (after archiving),
let's rewrite the file with COW enabled. This also takes care of
properly defragmenting the file.

With zstd compression level 3, journal files are compressed to 12%
of their original size with default journal settings.

As rewriting the file might take a while since we also do an fsync()
after the rewrite, this work is done in the offline thread to avoid
blocking the journald event loop.