um: fix sparse warnings from regset refactor

Some variables were not tagged with __user and another was not marked as
static even though it should be.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202410280655.gOlEFwdG-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202410281821.WSPsAwq7-lkp@intel.com/


Fixes: 3f17fed2 ("um: switch to regset API and depend on XSTATE")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20241031142017.430420-1-benjamin@sipsolutions.net


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