m68k: implement __clear_user(), which is needed by fs/signalfd.c

Since we always let the MMU do all checking, clear_user() and __clear_user()
are identical. The old clear_user() is renamed to __clear_user() for
consistency.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>