fsstress: avoid splice_f generating too large sparse file

fsstress: avoid splice_f generating too large sparse file

Thanks to Darrick J. Wong find this issue! Current splice_f generates
file offset as below:

  lr = ((int64_t)random() << 32) + random();
  off2 = (off64_t)(lr % maxfsize);

It generates a pseudorandom 64-bit candidate offset for the
destination file where we'll land the splice data, and then caps the
offset at maxfsize (which is 2^63- 1 on x64), which effectively means
that the data will appear at a very high file offset which creates
large (sparse) files very quickly.

That's not what we want, and some case likes shared/009 will take
forever to run md5sum on lots of huge files.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>