um: Do not flush MM in flush_thread

There should be no need to flush the memory in flush_thread. Doing this
likely worked around some issue where memory was still incorrectly
mapped when creating or cloning an MM.

With the removal of the special clone path, that isn't relevant anymore.
However, add the flush into MM initialization so that any new userspace
MM is guaranteed to be clean.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20240703134536.1161108-10-benjamin@sipsolutions.net


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