Quiet mkfs.ext2 output

The -q option should quiet the mkfs.ext2 output but certain
versions of e2fsprogs appear to ignore it. This can result in
an extra 'done' message in the test output. To keep this noise
from distracting just direct stdout to /dev/null.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>