userfaultfd: convert to ->read_iter()

Rather than use the older style ->read() hook, use ->read_iter() so that
userfaultfd can support both O_NONBLOCK and IOCB_NOWAIT for non-blocking
read attempts.

Split the fd setup into two parts, so that userfaultfd can mark the file
mode with FMODE_NOWAIT before installing it into the process table. With
that, we can also defer grabbing the mm until we know the rest will
succeed, as the fd isn't visible before then.

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