doc: Added separate troubleshooting for MDS and Ceph FS.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>

diff --git a/doc/cephfs/troubleshooting.rst b/doc/cephfs/troubleshooting.rst
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+=================
+ Troubleshooting
+=================
+
+
+Mount 5 Error
+=============
+
+A mount 5 error typically occurs if a MDS server is laggy or if it crashed.
+Ensure at least one MDS is up and running, and the cluster is ``active +
+healthy``. 
+
+
+Mount 12 Error
+==============
+
+A mount 12 error with ``cannot allocate memory`` usually occurs if you  have a
+version mismatch between the :term:`Ceph Client` version and the :term:`Ceph
+Storage Cluster` version. Check the versions using:: 
+
+       ceph -v
+       
+If the Ceph Client is behind the Ceph cluster, try to upgrade it:: 
+
+       sudo apt-get update && sudo apt-get install ceph-common 
+
+You may need to uninstall, autoclean and autoremove ``ceph-common`` 
+and then reinstall it so that you have the latest version.
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