IB/mlx5: Lock QP during page fault handling

When page fault event for a WQE arrives, the event data contains the
resource (e.g. QP) number which will later be used by the page fault
handler to retrieve the resource. Meanwhile, another context can destroy
the resource and cause use-after-free. To avoid that, take a reference on the
resource when handler starts and release it when it ends.

Page fault events for RDMA operations don't need to be protected because
the driver doesn't need to access the QP in the page fault handler.

Fixes: d9aaed83

 ("{net,IB}/mlx5: Refactor page fault handling")
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>