powerpc: use for_each_sg()

This replaces the plain loop over the sglist array with for_each_sg()
macro which consists of sg_next() function calls.  Since powerpc does
select ARCH_HAS_SG_CHAIN, it is necessary to use for_each_sg() in order
to loop over each sg element.  This also help find problems with drivers
that do not properly initialize their sg tables when CONFIG_DEBUG_SG is
enabled.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>