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.

https://lkml.org/lkml/2021/11/5/95

Signed-off-by: Sjoerd Simons sjoerd@collabora.com Signed-off-by: Ritesh Raj Sarraf ritesh.sarraf@collabora.com