io_uring/net: fix io_req_post_cqe abuse by send bundle

In the Linux kernel, the following vulnerability has been resolved:

io_uring/net: fix io_req_post_cqe abuse by send bundle

[ 114.987980][ T5313] WARNING: CPU: 6 PID: 5313 at io_uring/io_uring.c:872 io_req_post_cqe+0x12e/0x4f0

[ 114.991597][ T5313] RIP: 0010:io_req_post_cqe+0x12e/0x4f0

[ 115.001880][ T5313] Call Trace:

[ 115.002222][ T5313] <TASK>

[ 115.007813][ T5313] io_send+0x4fe/0x10f0

[ 115.009317][ T5313] io_issue_sqe+0x1a6/0x1740

[ 115.012094][ T5313] io_wq_submit_work+0x38b/0xed0

[ 115.013223][ T5313] io_worker_handle_work+0x62a/0x1600

[ 115.013876][ T5313] io_wq_worker+0x34f/0xdf0

As the comment states, io_req_post_cqe() should only be used by

multishot requests, i.e. REQ_F_APOLL_MULTISHOT, which bundled sends are

not. Add a flag signifying whether a request wants to post multiple

CQEs. Eventually REQ_F_APOLL_MULTISHOT should imply the new flag, but

that's left out for simplicity.