Btrfs: fix invalid dereference in btrfs_retry_endio

When doing directIO repair, we have this oops:

[ 1458.532816] general protection fault: 0000 [#1] SMP
...
[ 1458.536291] Workqueue: btrfs-endio-repair btrfs_endio_repair_helper [btrfs]
[ 1458.536893] task: ffff88082a42d100 task.stack: ffffc90002b3c000
[ 1458.537499] RIP: 0010:btrfs_retry_endio+0x7e/0x1a0 [btrfs]
...
[ 1458.543261] Call Trace:
[ 1458.543958]  ? rcu_read_lock_sched_held+0xc4/0xd0
[ 1458.544374]  bio_endio+0xed/0x100
[ 1458.544750]  end_workqueue_fn+0x3c/0x40 [btrfs]
[ 1458.545257]  normal_work_helper+0x9f/0x900 [btrfs]
[ 1458.545762]  btrfs_endio_repair_helper+0x12/0x20 [btrfs]
[ 1458.546224]  process_one_work+0x34d/0xb70
[ 1458.546570]  ? process_one_work+0x29e/0xb70
[ 1458.546938]  worker_thread+0x1cf/0x960
[ 1458.547263]  ? process_one_work+0xb70/0xb70
[ 1458.547624]  kthread+0x17d/0x180
[ 1458.547909]  ? kthread_create_on_node+0x70/0x70
[ 1458.548300]  ret_from_fork+0x31/0x40

It turns out that btrfs_retry_endio is trying to get inode from a directIO
page.

This fixes the problem by using the saved inode pointer, done->inode.
btrfs_retry_endio_nocsum has the same problem, and it's fixed as well.

Also cleanup unused @start (which is too trivial for a separate patch).

Cc: David Sterba <dsterba@suse.cz>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -7910,7 +7910,6 @@ struct btrfs_retry_complete {
 static void btrfs_retry_endio_nocsum(struct bio *bio)
 {
 	struct btrfs_retry_complete *done = bio->bi_private;
-	struct inode *inode;
 	struct bio_vec *bvec;
 	int i;
 
@@ -7918,12 +7917,12 @@ static void btrfs_retry_endio_nocsum(struct bio *bio)
 		goto end;
 
 	ASSERT(bio->bi_vcnt == 1);
-	inode = bio->bi_io_vec->bv_page->mapping->host;
-	ASSERT(bio->bi_io_vec->bv_len == btrfs_inode_sectorsize(inode));
+	ASSERT(bio->bi_io_vec->bv_len == btrfs_inode_sectorsize(done->inode));
 
 	done->uptodate = 1;
 	bio_for_each_segment_all(bvec, bio, i)
-	clean_io_failure(BTRFS_I(done->inode), done->start, bvec->bv_page, 0);
+		clean_io_failure(BTRFS_I(done->inode), done->start,
+				 bvec->bv_page, 0);
 end:
 	complete(&done->done);
 	bio_put(bio);
@@ -7986,9 +7985,7 @@ static void btrfs_retry_endio(struct bio *bio)
 {
 	struct btrfs_retry_complete *done = bio->bi_private;
 	struct btrfs_io_bio *io_bio = btrfs_io_bio(bio);
-	struct inode *inode;
 	struct bio_vec *bvec;
-	u64 start;
 	int uptodate;
 	int ret;
 	int i;
@@ -7998,11 +7995,8 @@ static void btrfs_retry_endio(struct bio *bio)
 
 	uptodate = 1;
 
-	start = done->start;
-
 	ASSERT(bio->bi_vcnt == 1);
-	inode = bio->bi_io_vec->bv_page->mapping->host;
-	ASSERT(bio->bi_io_vec->bv_len == btrfs_inode_sectorsize(inode));
+	ASSERT(bio->bi_io_vec->bv_len == btrfs_inode_sectorsize(done->inode));
 
 	bio_for_each_segment_all(bvec, bio, i) {
 		ret = __readpage_endio_check(done->inode, io_bio, i,