provide a sane discard_granularity default

[Christoph Hellwig <hch@lst.de>]

Hi Jens,

this series defaults the discard_granularity to the sector size as
that is a very logical default for devices that have no further
restrictions.  This removes the need for trivial drivers to set
a discard granularity and cleans up the interface.

Diffstat:
 arch/um/drivers/ubd_kern.c    |    1 -
 block/blk-merge.c             |    6 +-----
 block/blk-settings.c          |    5 ++++-
 drivers/block/nbd.c           |    6 +-----
 drivers/block/null_blk/main.c |    1 -
 drivers/block/zram/zram_drv.c |    1 -
 drivers/md/bcache/super.c     |    1 -
 drivers/mtd/mtd_blkdevs.c     |    4 +---
 8 files changed, 7 insertions(+), 18 deletions(-)