fanotify: remove event FAN_DIR_MODIFY

fanotify: remove event FAN_DIR_MODIFY

It was never enabled in uapi and its functionality is about to be
superseded by events FAN_CREATE, FAN_DELETE, FAN_MOVE with group
flag FAN_REPORT_NAME.

Keep a place holder variable name_event instead of removing the
name recording code since it will be used by the new events.

Link: https://lore.kernel.org/r/20200708111156.24659-17-amir73il@gmail.com
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>