kcmp: Fix ret code comparison

In the early draft of kcmp syscall it has been returning
[-1|0|1] values but finally [0|1|2] were merged into the
kernel, but I forgot to update the criu code. The good
thing is that because we're using rbtree the kcmp results
are still sorted and tree is balanced but sometime we may
take a wrong branch generating new ID even if the object
is present in the tree which eventually may lead to dump
faulure.

Reported-by: Deyan Doychev <deyan@1h.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>