[CONNECTOR]: Bugfix for cn_call_callback()

When system under heavy stress and must allocate new work
instead of reusing old one, new work must use correct
completion callback.

Patch is based on Philipp's and Lars' work.
I only cleaned small stuff (and removed spaces instead of tabs).

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>