xfstests: speed up 227 by using preallocation

xfstests: speed up 227 by using preallocation

Test 227 (fsr test) creates fragmented fre space by doing lots of
small writes to sparse offsets ni a file. This seeks the disk heads
around a lot writing data. We don't need to write data - just
trigger allocation. Hence use preallocation instead of data writes
and run at allocation speed rather than data write speed.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>