simplify bdi unregistation

[Christoph Hellwig <hch@xxxxxx>]

Hi Andrew,

this series simplifies the BDI code to get tid of the magic
auto-unregister feature that hid a recent block layer refcounting
bug.

Diffstat:
 drivers/mtd/mtdcore.c |    1 +
 fs/super.c            |    3 +++
 include/linux/fs.h    |    1 +
 mm/backing-dev.c      |   17 +++++++----------
 4 files changed, 12 insertions(+), 10 deletions(-)