RDMA/iwcm: Use a default listen backlog if needed

If the user creates a listening cm_id with backlog of 0 the IWCM ends
up not allowing any connection requests at all.  The correct behavior
is for the IWCM to pick a default value if the user backlog parameter
is zero.

Lustre from version 1.8.8 onward uses a backlog of 0, which breaks
iwarp support without this fix.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>