generic/135: redirect stderr from _scratch_mkfs

generic/135: redirect stderr from _scratch_mkfs

In generic/135, benign warning message output from _scratch_mkfs on
stderr can cause a golden output mismatch, as can occur when testing
an ext4 bigalloc file system.  It's the only generic test that does
not also redirect stderr when directing _scratch_mkfs' stdout to
/dev/null.  Since error output from _scratch_mkfs doesn't seem
important to the overall intent of the test, there should be no harm
in making its stderr handling consistent.

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>