generic/564: fix copy_range -f availability test

generic/564: fix copy_range -f availability test

generic/564 wants to test for copy_range -f, but as it's implemented
it calls copy_range with a length of zero which will silently return
success from the VFS (at least on some kernels) even if the underlying
fs doesn't support it.

So patch this up 2 ways; perform the test with an explicit length
so it's not a no-op, and go ahead test copy_range w/o -f in the test
first just to be on the safe side (and for clearer failure messages.)

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>