fsstress: avoid infinite loops in clonerange_f

fsstress: avoid infinite loops in clonerange_f

Evidently ceph will report a 4M blocksize, which trips clonerange_f's
clumsy attempt to avoid reflinking an extent on top of itself.  The
original code assumed that "pick a random destination up to 1MB past the
end of the file" would suffice, but that clearly won't with a 4M
blocksize.  Instead, we'll change it to 1024*blksize.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Tested-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>