IPoIB/cm: Don't crash if remote side uses one QP for both directions

The IPoIB CM spec allows the use of a single connection in both
active->passive and passive->active directions.  The current Linux
code uses one connection for both directions, but if another node only
uses one connection for both directions, we oops when we try to look
up the passive connection.  Fix by checking that qp_context is
non-NULL before dereferencing it.

Signed-off-by: Michael S. Tsirkin <mst@dev.mellanox.co.il>