[SCSI] block: fix the partial mappings with struct rq_map_data

This fixes bio_copy_user_iov to properly handle the partial mappings
with struct rq_map_data (which only sg uses for now but st and osst
will shortly). It adds the offset member to struct rq_map_data and
changes blk_rq_map_user to update it so that bio_copy_user_iov can add
an appropriate page frame via bio_add_pc_page().

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

fs/bio.c

diff --git a/fs/bio.c b/fs/bio.c
index 356e742..13be075 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -788,6 +788,7 @@
 	int i, ret;
 	int nr_pages = 0;
 	unsigned int len = 0;
+	unsigned int offset = map_data ? map_data->offset & ~PAGE_MASK : 0;
 
 	for (i = 0; i < iov_count; i++) {
 		unsigned long uaddr;
@@ -814,12 +815,16 @@
 	bio->bi_rw |= (!write_to_vm << BIO_RW);
 
 	ret = 0;
-	i = 0;
-	if (map_data)
+
+	if (map_data) {
 		nr_pages = 1 << map_data->page_order;
+		i = map_data->offset / PAGE_SIZE;
+	}
 	while (len) {
 		unsigned int bytes = PAGE_SIZE;
 
+		bytes -= offset;
+
 		if (bytes > len)
 			bytes = len;
 
@@ -841,10 +846,11 @@
 			}
 		}
 
-		if (bio_add_pc_page(q, bio, page, bytes, 0) < bytes)
+		if (bio_add_pc_page(q, bio, page, bytes, offset) < bytes)
 			break;
 
 		len -= bytes;
+		offset = 0;
 	}
 
 	if (ret)