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.

tmpfiles.d/journal-nocow.conf

@@ -0,0 +1,27 @@
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+# See tmpfiles.d(5) for details
+
+# Set the NOCOW attribute for directories of journal files. This flag
+# is inheredited by their new files and sub-directories. Matters only
+# for btrfs filesystems.
+#
+# WARNING: Enabling the NOCOW attribute improves journal performance
+#     substantially, but also disables the btrfs checksum logic. In
+#     btrfs RAID filesystems the checksums are needed for rebuilding
+#     corrupted files. Without checksums such rebuilds are not
+#     possible.
+#
+# In a single-disk filesystem (or a filesystem without redundancy)
+# enabling the NOCOW attribute for journal files is safe, because
+# they have their own checksums and a rebuilding wouldn't be possible
+# in any case.
+
+h /var/log/journal - - - - +C
+h /var/log/journal/%m - - - - +C
+h /var/log/journal/remote - - - - +C