fscrypt: pass a byte offset to fscrypt_generate_dun

fscrypt: pass a byte offset to fscrypt_generate_dun

Logical offsets into an inode are usually expressed as bytes in the VFS.
Switch fscrypt_generate_dun to that convention and remove the
ci_data_units_per_block_bits member in struct fscrypt_inode_info that
was only used to cache the DUN shift based on the logical block size
granularity.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20260302141922.370070-7-hch@lst.de
Signed-off-by: Eric Biggers <ebiggers@kernel.org>