CVE-2026-74334: 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 reregmr, 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 rdmarestracksync() operation which is sort of like flushworkqueue() or synchronizeirq(): 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.