lib: scatterlist: move SG pool code from SCSI driver to lib/sg_pool.c

Now it's ready to move the mempool based SG chained allocator code from
SCSI driver to lib/sg_pool.c, which will be compiled only based on a Kconfig
symbol CONFIG_SG_POOL.

SCSI selects CONFIG_SG_POOL.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lin <ming.l@ssi.samsung.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

include/scsi/scsi.h

@@ -17,25 +17,6 @@ enum scsi_timeouts {
 	SCSI_DEFAULT_EH_TIMEOUT		= 10 * HZ,
 };
 
-/*
- * The maximum number of SG segments that we will put inside a
- * scatterlist (unless chaining is used). Should ideally fit inside a
- * single page, to avoid a higher order allocation.  We could define this
- * to SG_MAX_SINGLE_ALLOC to pack correctly at the highest order.  The
- * minimum value is 32
- */
-#define SG_CHUNK_SIZE	128
-
-/*
- * Like SG_CHUNK_SIZE, but for archs that have sg chaining. This limit
- * is totally arbitrary, a setting of 2048 will get you at least 8mb ios.
- */
-#ifdef CONFIG_ARCH_HAS_SG_CHAIN
-#define SG_MAX_SEGMENTS	2048
-#else
-#define SG_MAX_SEGMENTS	SG_CHUNK_SIZE
-#endif
-
 /*
  * DIX-capable adapters effectively support infinite chaining for the
  * protection information scatterlist