[ARM] copypage: provide our own copy_user_highpage()

We used to override the copy_user_page() function.  However, this
is not only inefficient, it also causes additional complexity for
highmem support, since we convert from a struct page to a kernel
direct mapped address and back to a struct page again.

Moreover, with highmem support, we end up pointlessly setting up
kmap entries for pages which we're going to remap.  So, push the
kmapping down into the copypage implementation files where it's
required.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>