process_vm_access: switch to copy_page_to_iter/iov_iter_copy_from_user

... rather than open-coding those.  As a side benefit, we get much saner
loop calling those; we can just feed entire pages, instead of the "copy
would span the iovec boundary, let's do it in two loop iterations" mess.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>