Fix powerpc breakage in sg chaining code

Commit 78bdc310

 ("PPC: sg chaining
support") looks to have removed some unrelated ppc code.  Lets put it
back in.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>