io_uring: fix provided buffer import

io_import_iovec uses the s pointer, but this was changed immediately
after the iovec was re-imported and so it was imported into the wrong
place.

Change the ordering.

Fixes: 2be2eb0 ("io_uring: ensure reads re-import for selected buffers")
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220630132006.2825668-1-dylany@fb.com
[axboe: ensure we don't half-import as well]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -4318,18 +4318,19 @@ static int io_read(struct io_kiocb *req, unsigned int issue_flags)
 		if (unlikely(ret < 0))
 			return ret;
 	} else {
+		rw = req->async_data;
+		s = &rw->s;
+
 		/*
 		 * Safe and required to re-import if we're using provided
 		 * buffers, as we dropped the selected one before retry.
 		 */
-		if (req->flags & REQ_F_BUFFER_SELECT) {
+		if (io_do_buffer_select(req)) {
 			ret = io_import_iovec(READ, req, &iovec, s, issue_flags);
 			if (unlikely(ret < 0))
 				return ret;
 		}
 
-		rw = req->async_data;
-		s = &rw->s;
 		/*
 		 * We come here from an earlier attempt, restore our state to
 		 * match in case it doesn't. It's cheap enough that we don't