RDMA/srpt: fix integer overflow in immediate data length check

In the Linux kernel, the following vulnerability has been resolved: RDMA/srpt: fix integer overflow in immediate data length check imm_buf->len is a user-controlled uint32_t received from the network. Adding it to imm_data_offset without overflow checking allows a malicious initiator to send len=0xFFFFFFFF, causing req_size to wrap around to a small value, bypassing the bounds check, and subsequently passing a ~4GB length to sg_init_one(). Use check_add_overflow() to detect wrapping before the comparison.