[$] Resource limits in user namespaces

User namespaces provide a number ofinteresting challenges for the kernel. They give a user the illusion ofowning the system, but must still operate within the restrictions thatapply outside of the namespace. Resourcelimits represent one type of restriction that, it seems, is proving too restrictive for some users. Thispatch set from Alexey Gladkov attempts to address the problem by way ofa not-entirely-obvious approach.