Set DUN in dm-default_key

If the dun is left to the ICE block driver to set, any shuffling of
blocks will break encryption. This might happen in a number of scenarios
(e.g. using LVM) but specifically was happening here because of dm-bow
shuffling sector0 around.

Test: Coral boots with this patch, and if you turn on set_dun, requires
      a data wipe.
Bug: 137924328

Signed-off-by: Paul Lawrence <paullawrence@google.com>
[neersoni@codeaurora.org: In function drivers/md/dm_default_key.c:
dm_default_key_ctr_optional() initialized "struct dm_arg_set as", put braces
around if condition and removed empty line].
Change-Id: I6d440010c98ac47ec5237f6d9bb18de184a2c259
Signed-off-by: Neeraj Soni <neersoni@codeaurora.org>