sg_last() should use unsigned loop index variable

Clean up: fix a mixed sign comparison in sg_last() accidentally
introduced by commit 70eb8040.  The sign of the loop index variable
should match the sign of the "nents" argument.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jens Axboe <axboe@carl.home.kernel.dk>

include/linux/scatterlist.h

@@ -150,7 +150,7 @@ static inline struct scatterlist *sg_last(struct scatterlist *sgl,
 	struct scatterlist *ret = &sgl[nents - 1];
 #else
 	struct scatterlist *sg, *ret = NULL;
-	int i;
+	unsigned int i;
 
 	for_each_sg(sgl, sg, nents, i)
 		ret = sg;