RDMA/erdma: Hold QP references for AE and CM processing

In the Linux kernel, the following vulnerability has been resolved: RDMA/erdma: Hold QP references for AE and CM processing AE QP fatal events and iWARP CM paths load QPs from dev->qp_xa and then use or reference them outside the xarray lock. erdma_destroy_qp() can drop the destroy-path reference and free QP resources while such a lookup is in flight. Add erdma_qp_get_by_qpn() to acquire a kref under the xarray lock with kref_get_unless_zero(). Remove the QP from the xarray before dropping the destroy-path reference so no new lookup can acquire it while destruction waits for existing users.