namespaces: Use long value to check for UINT_MAX

We have a condition

	BUG_ON(kid > UINT_MAX);

but kid is unsigned int so it's never bigger than UINT_MAX,
use unsigned long instead.

CID 1042296

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>