io_uring: fix poll file assign deadlock

We pass "unlocked" into io_assign_file() in io_poll_check_events(),
which can lead to double locking.

Fixes: 6bf9c47a ("io_uring: defer file assignment")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/2476d4ae46554324b599ee4055447b105f20a75a.1649862516.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>