[IB] ib_umad: fix crash when freeing send buffers

The conversion of user_mad.c to the new MAD send API was slightly off:
in a few places, we used packet->msg instead of packet->msg->mad when
referring to the actual data buffer, which ended up corrupting the
underlying data structure and crashing when we free an invalid pointer.

Signed-off-by: Roland Dreier <rolandd@cisco.com>