md: dm-default-key: Override dun value for selected bios

Over ride dun values for file encrypted bios only when 'set_dun' flag
is set, file system is ext4 and content-encryption mode is
FS_CRYPT_PRIVATE.

Change-Id: I3340f29c71dbfb6006450415a718b661e49a6ed0
Signed-off-by: Neeraj Soni <neersoni@codeaurora.org>