xfs/167: need at least 10GB of scratch space to run

xfs/167: need at least 10GB of scratch space to run

When running on a ramdisk, the fsstress background workload consumes
a GB of disk space every 5 seconds. This leads to the test failing
with ENOSPC because the test file cannot be created due otthe
background load cosuming it all. Hence don't run this test unless
the scratch device is large enough not to hit ENOSPC conditions.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>