Generate random data only for buffers with sizes less than FAST_SIZE
If a size of buffer is more that FAST_SIZE, the first FAST_SIZE bytes
are filled by random generator and then this chunk is used as pattern
for all other chunks.

With out this patch:
$ time bash -x test/zdtm.sh static/maps04
real	0m16.777s
user	0m0.054s
sys	0m0.724s

With this patch:
$ time bash -x test/zdtm.sh static/maps04
real	0m1.865s
user	0m0.128s
sys	0m0.745s
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>