Fix: update writeback instrumentation for kernel 4.0+

Fix: update writeback instrumentation for kernel 4.0+

Introduced by Linux kernel commit:

commit 0ae45f63d4ef8d8eeec49c7d8b44a1775fff13e8
Author: Theodore Ts'o <tytso@mit.edu>
Date:   Mon Feb 2 00:37:00 2015 -0500

    vfs: add support for a lazytime mount option

And:

commit de1414a654e66b81b5348dbc5259ecf2fb61655e
Author: Christoph Hellwig <hch@lst.de>
Date:   Wed Jan 14 10:42:36 2015 +0100

    fs: export inode_to_bdi and use it in favor of mapping->backing_dev_info

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>