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>

fs/io_uring.c

@@ -3193,7 +3193,7 @@ static void io_req_map_rw(struct io_kiocb *req, const struct iovec *iovec,
 	rw->free_iovec = iovec;
 	rw->bytes_done = 0;
 	/* can only be fixed buffers, no need to do anything */
-	if (iter->type == ITER_BVEC)
+	if (iov_iter_is_bvec(iter))
 		return;
 	if (!iovec) {
 		unsigned iov_off = 0;