tmpfiles: Add +C attrib to the journal files directories

Add the +C file attribute (NOCOW) to the journal directories, so that
the flag is inherited automatically for new journal files created in
them. The journal write pattern is problematic on btrfs file systems as
it results in badly fragmented files when copy-on-write (COW) is used:
the performances decreases substantially over time.

To avoid this issue, this tmpfile.d snippet sets the NOCOW attribute to
the journal files directories, so newly created journal files inherit
the NCOOW attribute that disables copy-on-write.

Be aware that the NOCOW file attribute also disables btrfs checksumming
for these files, and thus prevents btrfs from rebuilding corrupted files
on a RAID filesystem.

In a single disk filesystems (or filesystems without redundancy) it is
safe to use the NOCOW flags without drawbacks, since the journal files
contain their own checksumming.

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