ssa.c: Add support for incoming "set parent" MADs if already connected

Such "set parent" MADs may arrive, as a result of activating
a rebalancing mechanism that might assign downstream node
with a new parent in order to make distribution tree more
balanced in terms of fanout for downstream connections.

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>