xfs/042: reduce runtime of the test

xfs/042: reduce runtime of the test

xfs/042 is really an xfs_fsr test, and it only writes about 60MB of
data. however, because it is trying to write lots of data in ENOSPC
conditions, it can take a long time as XFS does flushes to maximise
space usage at ENOSPC. e.g. on a slow 1p VM:

 xfs/042 426s ... 425s

It takes a long time to write a small amount of data. To avoid this
slow flushing problem, create the fragmented files with fallocate()
rather than write(), which fails much faster as there is no dirty
data to flush before retrying the allocation and failing. THis
results in:

 xfs/042 425s ... 11s

A massive reduction in runtime, such that we can consider putting
xfs/042 into the quick group....

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>