dump: do not fail dump when robust_lists are disabled

Robust lists may be disabled, for example if the "futex_cmpxchg_enabled"
variable in the kernel is unset.

Detect that case by checking that both "get_robust_list" and "set_robust_list"
syscalls return ENOSYS and do not make criu dump fail in that case, but simply
assume an empty list, which is consistent with the syscalls not being
available.

Tested: Successfully ran the zdtm test suite on a kernel where the
"get_robust_list" and "set_robust_list" syscalls are disabled.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>