IB/mlx4: Fix max_wqe capacity reported from query device

1. Limit the max number of WQEs per QP reported when querying the
   device, so that ib_create_qp() will not fail for a QP size that the
   device claimed to support due to additional headroom WQEs being
   allocated.

2. Limit qp resources accepted for ib_create_qp() to the limits
   reported in ib_query_device().  In kernel space, make sure that the
   limits returned to the caller following qp creation also lie within
   the reported device limits. For userspace, report as before, and do
   adjustment in libmlx4 (so as not to break ABI).

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Sagi Grimberg <sagig@mellanox.co.il>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>