tasks: more substantial IO for journal migration

...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>
Merge pull request #143 from ceph/wip-migration-test

tasks: more substantial IO for journal migration

Reviewed-by: Greg Farnum <greg@inktank.com>