btrfs: get bdev from latest_dev for dio bh_result

To remove use of extent_map::bdev we need to find a replacement, and the
latest_bdev is the only one we can use here, because inode::i_bdev and
superblock::s_bdev are NULL.

The DIO code uses bdev in two places:

* to read blocksize to perform alignment checks in
  do_blockdev_direct_IO, but we do them in btrfs code before any call to
  DIO

* in the following call chain:

  do_direct_IO
    get_more_blocks
     sdio->get_block() <-- this is btrfs_get_blocks_direct

  subsequently the map_bh->b_dev member is used in clean_bdev_aliases
  and dio_new_bio to set the bio's bdev to that of the buffer_head.
  However, because we have provided a submit function dio_bio_submit
  calls our submission function and ignores the bdev.

So it's safe to pass any valid bdev that's used within the filesystem.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -7662,6 +7662,8 @@ static int btrfs_get_blocks_direct_read(struct extent_map *em,
 					struct inode *inode,
 					u64 start, u64 len)
 {
+	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
+
 	if (em->block_start == EXTENT_MAP_HOLE ||
 			test_bit(EXTENT_FLAG_PREALLOC, &em->flags))
 		return -ENOENT;
@@ -7671,7 +7673,7 @@ static int btrfs_get_blocks_direct_read(struct extent_map *em,
 	bh_result->b_blocknr = (em->block_start + (start - em->start)) >>
 		inode->i_blkbits;
 	bh_result->b_size = len;
-	bh_result->b_bdev = em->bdev;
+	bh_result->b_bdev = fs_info->fs_devices->latest_bdev;
 	set_buffer_mapped(bh_result);
 
 	return 0;
@@ -7754,7 +7756,7 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map,
 	bh_result->b_blocknr = (em->block_start + (start - em->start)) >>
 		inode->i_blkbits;
 	bh_result->b_size = len;
-	bh_result->b_bdev = em->bdev;
+	bh_result->b_bdev = fs_info->fs_devices->latest_bdev;
 	set_buffer_mapped(bh_result);
 
 	if (!test_bit(EXTENT_FLAG_PREALLOC, &em->flags))