bdi: fix use-after-free for dev_name(bdi->dev) v3 (resend)

[Christoph Hellwig]

Hi Jens,

can you pick up this series?

the first three patches are my take on the proposal from Yufen Yu
to fix the use after free of the device name of the bdi device.

The rest is vaguely related cleanups.

Changes since v2:
 - switch vboxsf to a shorter bdi name

Changes since v1:
 - use a static dev_name buffer inside struct backing_dev_info