block, ice, cfq: support back/front merges and update req->__dun

<4>[136765.870673] c6   4967 ------------[ cut here ]------------
<4>[136765.870717] c6   4967 WARNING: at block/blk-core.c:1534
<4>[136765.871328] c6   4967 [<ffffff8aaceddec0>] bio_attempt_front_merge+0x23c/0x280
<4>[136765.871335] c6   4967 [<ffffff8aacedd16c>] blk_queue_bio+0x314/0xaa8
<4>[136765.871344] c6   4967 [<ffffff8aaced6f68>] generic_make_request+0xfc/0x330
<4>[136765.871351] c6   4967 [<ffffff8aaced6cec>] submit_bio+0x15c/0x2dc
<4>[136765.871362] c6   4967 [<ffffff8aacdfeaf8>] __submit_bio+0x46c/0x4e0
<4>[136765.871371] c6   4967 [<ffffff8aace0abe8>] f2fs_mpage_readpages+0x57c/0x7e4

Assuming that this encrypted bio is mergeable,

blk_queue_bio()
  blk_attempt_plug_merge() = false, given by blk_try_merge()=ELEVATOR_NO_MERGE
  el_ret = elv_merge()
      if (q->last_merge)
          elv_bio_merge_ok() = true
          blk_try_merge() = ELEVATOR_NO_MERGE;
      }
      __rq = elv_rqhash_find();
      if (__rq = false && elv_bio_merge_ok() = true)
      e->type->ops.elevator_merge_fn() -- cfq_merge()
          elv_bio_merge_ok() = true
      return ELEVATOR_FRONT_MERGE
  if (el_ret == ELEVATOR_FRONT_MERGE)
     bio_attempt_front_merge()
         WARN_ON(dun exists)

So, it should update req->__dun having correct ICE configuration.

Bug: 118824609
Bug: 119454809
Change-Id: Ic79f9474b56b2b911c9083c3a80f3e686bb432bd
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
[stummala@codeaurora.org: Fix merge conflicts]
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>