mpage: make __mpage_writepage() honor cgroup writeback

__mpage_writepage() is used to implement mpage_writepages() which in
turn is used for ->writepages() of various filesystems.  All writeback
logic is now updated to handle cgroup writeback and the block cgroup
to issue IOs for is encoded in writeback_control and can be retrieved
from the inode; however, __mpage_writepage() currently ignores the
blkcg indicated by the inode and issues all bio's without explicit
blkcg association.

This patch updates __mpage_writepage() so that the issued bio's are
associated with inode_to_writeback_blkcg_css(inode).

v2: Updated for per-inode wb association.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jens Axboe <axboe@fb.com>

fs/mpage.c

@@ -605,6 +605,8 @@ alloc_new:
 				bio_get_nr_vecs(bdev), GFP_NOFS|__GFP_HIGH);
 		if (bio == NULL)
 			goto confused;
+
+		bio_associate_blkcg(bio, inode_to_wb_blkcg_css(inode));
 	}
 
 	/*