scatterlist: allow limited chaining without ARCH_HAS_SG_CHAIN

There are a couple of uses of struct scatterlist that never go to
the dma_map_sg() helper and thus don't care about ARCH_HAS_SG_CHAIN
which indicates that we can map chained S/G list.

The most important one is the crypto code, which currently has
to open code a few helpers to always allow chaining.  This patch
removes a few #ifdef ARCH_HAS_SG_CHAIN statements so that we can
switch the crypto code to these common helpers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/scatterlist.h

@@ -161,10 +161,6 @@ static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
 static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,
 			    struct scatterlist *sgl)
 {
-#ifndef CONFIG_ARCH_HAS_SG_CHAIN
-	BUG();
-#endif
-
 	/*
 	 * offset and length are unused for chain entry.  Clear them.
 	 */

lib/scatterlist.c

@@ -105,16 +105,12 @@ EXPORT_SYMBOL(sg_nents_for_len);
  **/
 struct scatterlist *sg_last(struct scatterlist *sgl, unsigned int nents)
 {
-#ifndef CONFIG_ARCH_HAS_SG_CHAIN
-	struct scatterlist *ret = &sgl[nents - 1];
-#else
 	struct scatterlist *sg, *ret = NULL;
 	unsigned int i;
 
 	for_each_sg(sgl, sg, nents, i)
 		ret = sg;
 
-#endif
 #ifdef CONFIG_DEBUG_SG
 	BUG_ON(sgl[0].sg_magic != SG_MAGIC);
 	BUG_ON(!sg_is_last(ret));