Currently we use a systemd mount (var-lib-systemd-coredump.mount) to mount
default coredump directory (/var/lib/systemd/coredump) to
(/var/lib/scylla/coredump). The /var/lib/scylla had been mounted to a big
storage, so we will have enough space for coredump after the mount.
Currently in coredump_setup, we only enabled var-lib-systemd-coredump.mount,
but not start it. The directory won't be mounted after coredump_setup, so the
coredump will still be saved to default coredump directory.
The mount will only effect after reboot.
Fixes#6566
(cherry picked from commit abf246f6e5)