hostfs: Fix writeback of dirty pages

Hostfs was not setting up the backing device information, which means it
uses the noop bdi. The noop bdi does not have the writeback capability
enabled, which in turns means  dirty pages never got written back to
storage.

In other words programs using mmap to write to files on  hostfs never
actually got their data written out...

Fix this by simply setting up the bdi with default settings as all the
required code for writeback is already in place.

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
Reviewed-by: Christopher Obbard <chris.obbard@collabora.com>
Tested-by: Ritesh Raj Sarraf <ritesh@collabora.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>