f2fs/fscrypt-ice: allow merging some IOs

This patch enables IO merge for ICE-based fscrypto by declaring bi_dun in
bi_iter. The reason we couldn't merge it turns out elv_merge() just bypasses
checking crypto merge condition.

So, now we can remove setting REQ_NOMERGE_FLAGS by F2FS.

Note that, there's no written data change in disk, which doesn't require
wiping userdata.

Change-Id: I1a1ed631e3486291a456b74b1fb3afe77b2a0f2d
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Signed-off-by: Neeraj Soni <neersoni@codeaurora.org>