m68k: Remove big kernel lock in cache flush code

The cache flush code doesn't need a lock, so we can remove the use of the BKL.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Andreas Schwab <schwab@linux-m68k.org>