fix memcpy_to_page and memzero_page

[Christoph Hellwig]

Hi all,

we need to call flush_dcache_page on any kmapped page, and on any page
we write to that may be mapped into the page cache, which can happen
quite frequently for anything we use raw pages for.

Patch 1 make sure memcpy_to_page and memzero_page to do the right thing
for that, and patch 2 fixes up the lost switch to kmap_local_page for
memzero_page.