With dax we cannot deal with readpage() etc. So, we create a
funciton callback to perform the file data comparison and pass
it to generic_remap_file_range_prep() so it can use iomap-based
functions.

In order to be compatible with future work of virtio-pmem,
deny mixed-mode file comparison.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>