io_uring: fix ITER_BVEC check

iov_iter::type is a bitmask that also keeps direction etc., so it
shouldn't be directly compared against ITER_*. Use proper helper.

Fixes: ff6165b2 ("io_uring: retain iov_iter state over io_read/io_write calls")
Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Cc: <stable@vger.kernel.org> # 5.9
Signed-off-by: Jens Axboe <axboe@kernel.dk>