renaming session parameter form user_context to private_data.
The parameter references a user-controlled data buffer.
The contents of the buffer are copied and transparently passed to the
remote side as part of the communication request.
May be NULL if private_data is not required.

Change-Id: Iaf544f02b346c090dd84a35d90387f6ef399610f
Signed-off-by: Eyal Salomon <esalomon@mellanox.com>