Ensure that adding a user to the list of users of a specific music on hold

Ensure that adding a user to the list of users of a specific music on hold
class is not done at the same time as any of the other operations on this list
to prevent list corruption.  Using the global moh_data lock for this is not
ideal, but it is what is used to protect these lists everywhere else in the
module, and I am only changing what is necessary to fix the bug.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.2@75059 65c4cc65-6c06-0410-ace0-fbb531ad65f3