RDMA/rtrs-srv: Fix integer underflow in process_read and process_write

In the Linux kernel, the following vulnerability has been resolved: RDMA/rtrs-srv: Fix integer underflow in process_read and process_write usr_len is read from a network-supplied message field (le16_to_cpu) and used to compute data_len = off - usr_len without validating that usr_len <= off. A malicious RDMA client can send usr_len > off causing an integer underflow, resulting in data_len wrapping to a huge size_t value which is then passed to the rdma_ev callback as a memory length, leading to out-of-bounds memory access. Fix by reading and validating usr_len <= off before rtrs_srv_get_ops_ids() in both process_read() and process_write(), ensuring the early return path acquires no reference and has no resource leak.