um: Make mm_list and mm_list_lock static

They are only used within mmu.c. Make them static.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250708090403.1067440-3-tiwei.bie@linux.dev


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