IB/mad: initialize mad_agent_priv before putting on lists

There is a potential race in ib_register_mad_agent() where the struct
ib_mad_agent_private is not fully initialized before it is added to
the list of agents per IB port. This means the ib_mad_agent_private
could be seen before the refcount, spin locks, and linked lists are
initialized.  The fix is to initialize the structure earlier.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>