IB/mad: Verify mgmt class in received MADs

If a received MAD contains an invalid or reserved mgmt class, we will
attempt to access method_table outside of its range.  Add a check to
ensure that mgmt class falls within the handled range.

Found by code inspection.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>