Btrfs: fix crash on endio of reading corrupted block

The crash is

------------[ cut here ]------------
kernel BUG at fs/btrfs/extent_io.c:2124!
[...]
Workqueue: btrfs-endio normal_work_helper [btrfs]
RIP: 0010:[<ffffffffa02d6055>]  [<ffffffffa02d6055>] end_bio_extent_readpage+0xb45/0xcd0 [btrfs]

This is in fact a regression.

It is because we forgot to increase @offset properly in reading corrupted block,
so that the @offset remains, and this leads to checksum errors while reading
left blocks queued up in the same bio, and then ends up with hiting the above
BUG_ON.
Reported-by: NChris Murphy <lists@colorremedies.com>
Signed-off-by: NLiu Bo <bo.li.liu@oracle.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/extent_io.c

@@ -2532,6 +2532,7 @@ static void end_bio_extent_readpage(struct bio *bio, int err)
 					test_bit(BIO_UPTODATE, &bio->bi_flags);
 				if (err)
 					uptodate = 0;
+				offset += len;
 				continue;
 			}
 		}