As an optimization, don't encrypt blocks fully beyond i_size, since
those definitely won't need to be written out.  Also add a comment.

This is in preparation for allowing encryption on ext4 filesystems with
blocksize != PAGE_SIZE.

This is based on work by Chandan Rajendra.
Reviewed-by: Chandan Rajendra <chandan@linux.ibm.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>