dm-bufio: allow access to the partial buffer at the end of the device

If the device size is not divisible by the buffer size, dm-bufio would
return an I/O error when accessing the last partial buffer on the device.
This commit changes it so that it can access the partial buffer.

This change is needed to resolve a dm-buffered error when accessing the
last buffer on the device.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 drivers/md/dm-bufio.c |   15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

Index: linux-2.6/drivers/md/dm-bufio.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-bufio.c
+++ linux-2.6/drivers/md/dm-bufio.c
@@ -1361,7 +1361,7 @@ static void submit_io(struct dm_buffer *
 		      void (*end_io)(struct dm_buffer *, blk_status_t))
 {
 	unsigned int n_sectors;
-	sector_t sector;
+	sector_t sector, sector_limit;
 	unsigned int offset, end;
 
 	b->end_io = end_io;
@@ -1386,6 +1386,19 @@ static void submit_io(struct dm_buffer *
 		n_sectors = (end - offset) >> SECTOR_SHIFT;
 	}
 
+	sector_limit = bdev_nr_sectors(b->c->bdev);
+	if (unlikely(sector + n_sectors <= sector) ||
+	    unlikely(sector + n_sectors > sector_limit)) {
+		if (sector < sector_limit) {
+			n_sectors = sector_limit - sector;
+			if (op == REQ_OP_READ)
+				memset(b->data, 0, b->c->block_size);
+		} else {
+			b->end_io(b, BLK_STS_NOSPC);
+			return;
+		}
+	}
+
 	if (b->data_mode != DATA_MODE_VMALLOC)
 		use_bio(b, op, sector, n_sectors, offset);
 	else