um: Limit TASK_SIZE to the addressable range

We may have a TASK_SIZE from the host that is bigger than UML is able to
address with a three-level pagetable on 64-bit. Guard against that by
clipping the maximum TASK_SIZE to the maximum addressable area.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20240919124511.282088-8-benjamin@sipsolutions.net


Signed-off-by: Johannes Berg <johannes.berg@intel.com>