common/rc: fix _repair_scratch_fs error reporting

common/rc: fix _repair_scratch_fs error reporting

_dump_err2() helper was introduced over two years ago and it broke
the error reporting from _repair_scratch_fs.

Every test that runs:
_repair_scratch_fs >> $seqres.full

and expect to fail if anything is written to stderr is not working
as intended.

Reported-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>