iov_iter for ep_read_iter can be ITER_UBUF with io_uring.
In that case dup_iter() does not have to allocate iov and it can
return NULL. Fix the assumption by checking for iter_is_ubuf()
other wise ep_read_iter can treat this as failure and return -ENOMEM.

Fixes: 1e23db45 ("io_uring: use iter_ubuf for single range imports")
Signed-off-by: Sandeep Dhavale <dhavale@google.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20230401060509.3608259-3-dhavale@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>