io_uring: don't attempt to IOPOLL for MSG_RING requests

We gate whether to IOPOLL for a request on whether the opcode is allowed
on a ring setup for IOPOLL and if it's got a file assigned. MSG_RING
is the only one that allows a file yet isn't pollable, it's merely
supported to allow communication on an IOPOLL ring, not because we can
poll for completion of it.

Put the assigned file early and clear it, so we don't attempt to poll
for it.

Reported-by:  <syzbot+1a0a53300ce782f8b3ad@syzkaller.appspotmail.com>
Fixes: 3f1d52ab

 ("io_uring: defer msg-ring file validity check until command issue")
Signed-off-by: Jens Axboe <axboe@kernel.dk>