xfstests: use -F when making scratch ext4 fs

xfstests: use -F when making scratch ext4 fs

mkfs.ext4 will ask you if you are sure if you try to mkfs an entire disk instead
of just a partition, so any xfstest that uses a scratch device and you specify a
full disk will appear to hang because it's waiting for you to say yes.  This
fixes the problem by just forcing it to do its thing.

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>