generic: reorder output to stderr and stdout from _scratch_mkfs

generic: reorder output to stderr and stdout from _scratch_mkfs

The change fixes false positives from generic/554 and generic/565 tests,
when supplementary output of an mkfs utility to stderr apprears to be mixed
into the test output, which results into non-empty diff with the expected
output found in *.out files.

The change unifies _scratch_mkfs redirections among all generic tests, these
two modified tests are the only ones with the swapped order of output
redirections.

Signed-off-by: Vladimir Zapolskiy <vladimir@tuxera.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>