bcache/md: Use raid stripe size

Now that we've got code for raid5/6 stripe awareness, bcache just needs
to know about the stripes and when writing partial stripes is expensive
- we probably don't want to enable this optimization for raid1 or 10,
even though they have stripes. So add a flag to queue_limits.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

diff --cc block/blk-settings.c
Simple merge

diff --cc drivers/md/bcache/super.c
Simple merge

diff --cc drivers/md/raid5.c
Simple merge

diff --cc include/linux/blkdev.h
Simple merge