um: Make unscheduled_userspace_iterations static

It's only used within process.c. Make it static.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250708090403.1067440-2-tiwei.bie@linux.dev


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