IB/iser: Protect completion context active_qps update

As iser connections can share completion contexts, we need
to protect the active_qps update each time we set it.

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Acked-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>