bpf, sockmap: Several fixes to bpf_msg_pop_data

In the Linux kernel, the following vulnerability has been resolved: bpf, sockmap: Several fixes to bpf_msg_pop_data Several fixes to bpf_msg_pop_data, 1. In sk_msg_shift_left, we should put_page 2. if (len == 0), return early is better 3. pop the entire sk_msg (last == msg->sg.size) should be supported 4. Fix for the value of variable "a" 5. In sk_msg_shift_left, after shifting, i has already pointed to the next element. Addtional sk_msg_iter_var_next may result in BUG.