affs: use memcpy_to_page and remove replace kmap_atomic()

The use of kmap() is being deprecated in favor of kmap_local_page()
where it is feasible. For kmap around a memcpy there's a convenience
helper memcpy_to_page that also makes the flush_dcache_page() redundant.

CC: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: David Sterba <dsterba@suse.com>