IB/srp: Enable multiple connections to the same target

Enable multiple concurrent connections to the same SRP target:

1) Use port GUID instead of node GUID in the initiator port
   identifier.  This allows connections to be made from multiple HCA
   ports at the same time.
2) Let the user specify the identifier extention when adding the
   device.  This allows userspace to make multiple connections even
   from the same port, if it wants too.

Without this, only one connection can be made from any given HCA, even
if it has multiple ports, because we don't use multi-channel mode, so
targets will only allow one connection from a given initiator port ID.

Signed-off-by: Ishai Rabinovitz <ishai@mellanox.co.il>
Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>