This was only used in get_first_mon, which doesn't actually
need the parameter itself. Makes it easier to casually
use Filesystem from any place with a ctx to hand.
Signed-off-by: John Spray <john.spray@redhat.com>
...so that there will at least be multiple segments
in the log during the rewrite.
Also make the test stricter by checking that
cephfs-journal-tool can happily read the resulting
journal.
Signed-off-by: John Spray <john.spray@redhat.com>
