task/mds_journal_migration: fix cleanup

Previously this was remounting in the background
which meant that if a workunit was executed immediately
afterwards it could put files in the mount dir before
the FS was mounted, causing the mount to fail.

Signed-off-by: John Spray <john.spray@redhat.com>

teuthology/task/mds_journal_migration.py

@@ -101,5 +101,6 @@ def task(ctx, config):
     # Leave all MDSs and clients running for any child tasks
     for mount in ctx.mounts.values():
         mount.mount()
+        mount.wait_until_mounted()
 
     yield