IB/core: Validate route when we init ah

In order to make sure API users don't try to use SGIDs which don't
conform to the routing table, validate the route before searching
the RoCE GID table.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>