qa: optionally use old fs name during cephfs remount

This fixes errors caused by remount done by some tests (test_recovery_pool.py)
where the fs name is not given.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>

qa/tasks/cephfs/mount.py

@@ -41,6 +41,9 @@ class CephFSMount(object):
         raise NotImplementedError()
 
     def setupfs(self, name=None):
+        if name is None and self.fs is not None:
+            # Previous mount existed, reuse the old name
+            name = self.fs.name
         self.fs = Filesystem(self.ctx, name=name)
         log.info('Wait for MDS to reach steady state...')
         self.fs.wait_for_daemons()