IB/mad: Handle DR SMPs with a LID routed part

Fix handling of directed route SMPs with a beginning or ending LID
routed part.

Signed-off-by: Ralph Campbell <ralphc@pathscale.com>
Signed-off-by: Hal Rosenstock <halr@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>