ublk: handle ublk_set_auto_buf_reg() failure correctly in ublk_fetch()

If ublk_set_auto_buf_reg() fails, we need to unlock and return,
otherwise `ub->mutex` is leaked.

Fixes: 99c1e4e ("ublk: register buffer to local io_uring with provided buf index via UBLK_F_AUTO_BUF_REG")
Reported-by: Caleb Sander Mateos <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Caleb Sander Mateos <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: Ming Lei

drivers/block/ublk_drv.c

@@ -2075,7 +2075,7 @@ static int ublk_fetch(struct io_uring_cmd *cmd, struct ublk_queue *ubq,
 	if (ublk_support_auto_buf_reg(ubq)) {
 		ret = ublk_set_auto_buf_reg(cmd);
 		if (ret)
-			return ret;
+			goto out;
 	}
 
 	ublk_fill_io_cmd(io, cmd, buf_addr);