IB/core: Add support for multicast loopback blocking

This patch also adds a creation flag for QPs,
IB_QP_CREATE_MULTICAST_BLOCK_LOOPBACK, which when set means that
multicast sends from the QP to a group that the QP is attached to will
not be looped back to the QP's receive queue.  This can be used to
save receive resources when a consumer does not want a local copy of
multicast traffic; for example IPoIB must waste CPU time throwing away
such local copies of multicast traffic.

This patch also adds a device capability flag that shows whether a
device supports this feature or not.

Signed-off-by: Ron Livne <ronli@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>