um: Fix potential integer overflow during physmem setup

This issue happens when the real map size is greater than LONG_MAX,
which can be easily triggered on UML/i386.

Fixes: fe205bdd ("um: Print minimum physical memory requirement")
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20240916045950.508910-3-tiwei.btw@antgroup.com


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