scsi: lib/sg_pool.c: clear 'first_chunk' in case of no preallocation

If user doesn't ask to preallocate by passing zero 'nents_first_chunk' to
sg_alloc_table_chained, we need to make sure that 'first_chunk' is cleared.
Otherwise, __sg_alloc_table() still may think that the 1st SGL should be
from the preallocation.

Fixes the issue by clearing 'first_chunk' in sg_alloc_table_chained() if
'nents_first_chunk' is zero.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Ewan D. Milne <emilne@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Author: Ming Lei

lib/sg_pool.c

@@ -102,7 +102,9 @@ EXPORT_SYMBOL_GPL(sg_free_table_chained);
  *
  *  Description:
  *    Allocate and chain SGLs in an sg table. If @nents@ is larger than
- *    @nents_first_chunk a chained sg table will be setup.
+ *    @nents_first_chunk a chained sg table will be setup. @first_chunk is
+ *    ignored if nents_first_chunk <= 1 because user expects the SGL points
+ *    non-chain SGL.
  *
  **/
 int sg_alloc_table_chained(struct sg_table *table, int nents,
@@ -121,7 +123,7 @@ int sg_alloc_table_chained(struct sg_table *table, int nents,
 	}
 
 	/* User supposes that the 1st SGL includes real entry */
-	if (nents_first_chunk == 1) {
+	if (nents_first_chunk <= 1) {
 		first_chunk = NULL;
 		nents_first_chunk = 0;
 	}