Blackfin: SMP: fix cache flush loop

The recent commit (10774912647781) wasn't entirely correct.  While
it fixed some issues, it introduced others.  So pull in the fixes
from the public cache flush functions, and document why we need to
call things directly ourselves.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>