Since version 7 clients must use MAP_PRIVATE to map the keymap fd so we
can use memfd_create in os_ro_anonymous_file_get_ref using
RO_ANONYMOUS_FILE_MAPMODE_PRIVATE, for older version we use
RO_ANONYMOUS_FILE_MAPMODE_SHARED to be compatibile with MAP_SHARED.
Signed-off-by: Sebastian Wick <sebastian@sebastianwick.net>