journald: turn off COW for journal files on btrfs

btrfs' COW logic results in heavily fragment journal files, which is
detrimental for perfomance. Hence, turn off COW for journal files as we
create them.

Turning off COW comes at the cost of data integrity guarantees, but this
should be acceptable, given that we do our own checksumming, and
generally have a pretty conservative write pattern.

Also see discussion on linux-btrfs:

http://www.spinics.net/lists/linux-btrfs/msg41001.html

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