io_uring: fix queueing half-created requests

io_uring: fix queueing half-created requests

[   27.259845] general protection fault, probably for non-canonical address 0xdffffc0000000005: 0000 [#1] SMP KASAN PTI
[   27.261043] KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
[   27.263730] RIP: 0010:sock_from_file+0x20/0x90
[   27.272444] Call Trace:
[   27.272736]  io_sendmsg+0x98/0x600
[   27.279216]  io_issue_sqe+0x498/0x68d0
[   27.281142]  __io_queue_sqe+0xab/0xb50
[   27.285830]  io_req_task_submit+0xbf/0x1b0
[   27.286306]  tctx_task_work+0x178/0xad0
[   27.288211]  task_work_run+0xe2/0x190
[   27.288571]  exit_to_user_mode_prepare+0x1a1/0x1b0
[   27.289041]  syscall_exit_to_user_mode+0x19/0x50
[   27.289521]  do_syscall_64+0x48/0x90
[   27.289871]  entry_SYSCALL_64_after_hwframe+0x44/0xae

io_req_complete_failed() -> io_req_complete_post() ->
io_req_task_queue() still would try to enqueue hard linked request,
which can be half prepared (e.g. failed init), so we can't allow
that to happen.

Fixes: a8295b982c46d ("io_uring: fix failed linkchain code logic")
Reported-by: syzbot+f9704d1878e290eddf73@syzkaller.appspotmail.com
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/70b513848c1000f88bd75965504649c6bb1415c0.1630415423.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>