RDMA/nldev: Fix locking when accessing mr->pd

In the Linux kernel, the following vulnerability has been resolved: RDMA/nldev: Fix locking when accessing mr->pd Sashiko points out that, due to rereg_mr, the PD is actually variable and all the touches in nldev are racy. Use mr->device instead of mr->pd->device. Getting the PD restrack ID is more tricky. To avoid disturbing all the happy paths, add an rdma_restrack_sync() operation which is sort of like flush_workqueue() or synchronize_irq(): after it returns, all the old nldev touches to the mr are gone and everything sees the new PD. This makes it safe to reach into the PD pointer.

Where it's fixed