dm-thin does not free the discard_parent bio after all chained sub
bios finished. The following kmemleak report could be observed after
pool with discard_passdown option processes discard bios in
linux v4.11-rc7. To fix this, we drop the discard_parent bio reference
when its endio (passdown_endio) called.

unreferenced object 0xffff8803d6b29700 (size 256):
  comm "kworker/u8:0", pid 30349, jiffies 4379504020 (age 143002.776s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    01 00 00 00 00 00 00 f0 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff81a5efd9>] kmemleak_alloc+0x49/0xa0
    [<ffffffff8114ec34>] kmem_cache_alloc+0xb4/0x100
    [<ffffffff8110eec0>] mempool_alloc_slab+0x10/0x20
    [<ffffffff8110efa5>] mempool_alloc+0x55/0x150
    [<ffffffff81374939>] bio_alloc_bioset+0xb9/0x260
    [<ffffffffa018fd20>] process_prepared_discard_passdown_pt1+0x40/0x1c0 [dm_thin_pool]
    [<ffffffffa018b409>] break_up_discard_bio+0x1a9/0x200 [dm_thin_pool]
    [<ffffffffa018b484>] process_discard_cell_passdown+0x24/0x40 [dm_thin_pool]
    [<ffffffffa018b24d>] process_discard_bio+0xdd/0xf0 [dm_thin_pool]
    [<ffffffffa018ecf6>] do_worker+0xa76/0xd50 [dm_thin_pool]
    [<ffffffff81086239>] process_one_work+0x139/0x370
    [<ffffffff810867b1>] worker_thread+0x61/0x450
    [<ffffffff8108b316>] kthread+0xd6/0xf0
    [<ffffffff81a6cd1f>] ret_from_fork+0x3f/0x70
    [<ffffffffffffffff>] 0xffffffffffffffff

Cc: stable@vger.kernel.org
Signed-off-by: Dennis Yang <dennisyang@qnap.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>