um: remove file sync for stub data

There is no need to sync the stub code to "disk" for the other process
to see the correct memory. Drop the fsync there and remove the helper
function.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20241103150506.1367695-3-benjamin@sipsolutions.net


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