ib/mad: simplify snooping interface

ib/mad: simplify snooping interface

In preparation for exporting the kernel mad snooping capability
to user space, remove all code originally inserted as place holders
and simplify the mad snooping interface.

For performance reasons, we want to filter which mads are reported
to clients of the snooping interface at the lowest level, but we
also don't want to perform complex filtering at that level.
As a trade-off, we allow filtering based on mgmt_class, attr_id,
and mad request status.

The reasoning behind these choices are to allow a user to filter
traffic to a specific service (the SA or CM), for a well known
purpose (path record queries or multicast joins), or view only
operations that have failed.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>