block_dev: get bdev inode bdi directly from the block device

Directly grab the backing_dev_info from the request_queue instead of
detouring through the address_space.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NTejun Heo <tj@kernel.org>
Reviewed-by: NJan Kara <jack@suse.cz>
Signed-off-by: NJens Axboe <axboe@fb.com>

fs/fs-writeback.c

@@ -69,10 +69,10 @@ EXPORT_SYMBOL(writeback_in_progress);
 static inline struct backing_dev_info *inode_to_bdi(struct inode *inode)
 {
 	struct super_block *sb = inode->i_sb;
-
+#ifdef CONFIG_BLOCK
 	if (sb_is_blkdev_sb(sb))
-		return inode->i_mapping->backing_dev_info;
-
+		return blk_get_backing_dev_info(I_BDEV(inode));
+#endif
 	return sb->s_bdi;
 }