bcache: Use blkdev_issue_discard()

The old asynchronous discard code was really a relic from when all the
allocation code was asynchronous - now that allocation runs out of a
dedicated thread there's no point in keeping around all that complicated
machinery.

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

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

diff --cc drivers/md/bcache/bcache.h
Simple merge

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