f2fs, fscrypt: clean up ICE support

In preparation for adding the IV_INO_LBLK_32 IV generation method, stop
hard-coding the DUN generation.  Instead, make f2fs and fs/direct-io.c
use helper functions fscrypt_set_bio_crypt_ctx() and
fscrypt_mergeable_bio(), like blk-crypto in newer kernels.

No change is made to how the DUN and key are actually passed down to the
block layer.  So unlike real blk-crypto, fscrypt_set_bio_crypt_ctx()
only sets the DUN.  The key is still implicit via pfk_bio_get_inode().

Change-Id: I79bde6714ea710086345bfe3396cadad1a0204bb
Signed-off-by: Eric Biggers <ebiggers@google.com>
[neersoni@codeaurora.org: fix merge conflicts and compilation errors]
Signed-off-by: Neeraj Soni <neersoni@codeaurora.org>
[vagrawa@codeaurora.org: fix merge conflicts in fs/f2fs/data.c]
Signed-off-by: Vaibhav Agrawal <vagrawa@codeaurora.org>