Revert "bdi: add a ->dev_name field to struct backing_dev_info"

This reverts commit d6c434ae which is
commit 6bd87eec23cbc9ed222bed0f5b5b02bf300e9a8d upstream.

It breaks the Android KABI and is not needed in this branch at this
point in time.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I69a71f7ae3dca4c5bec52b31dcc082ad1c343220