cachefiles: Allow the cache to be non-root

Set mode 0600 on files in the cache so that cachefilesd can run as an
unprivileged user rather than leaving the files all with 0.  Directories
are already set to 0700.

Userspace then needs to set the uid and gid before issuing the "bind"
command and the cache must've been chown'd to those IDs.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
cc: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-erofs@lists.ozlabs.org
cc: linux-fsdevel@vger.kernel.org
Message-Id: <1853230.1684516880@warthog.procyon.org.uk>
Signed-off-by: Christian Brauner <brauner@kernel.org>