block/blk-iocost: ensure 'ret' is set on error

In case blkg_conf_open_bdev_frozen() fails, ioc_qos_write() jumps to the
error path without assigning a value to 'ret'. Ensure that it inherits
the error from the passed back error value.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202503200454.QWpwKeJu-lkp@intel.com/


Fixes: 9730763f ("block: correct locking order for protecting blk-wbt parameters")
Signed-off-by: Jens Axboe <axboe@kernel.dk>