Before this commit u2mfn got the address via kmap(get_user_pages(...)).
This has the problem that get_user_pages() does not work in some cases.
For example when qemu in a stubdom tries to get the mfn for a page which
is mapped from the target domain u2mfn fails because get_user_pages()
cannot be used for such mappings.
So get the address from the pte with apply_to_page_range(). Since we are
only interested in one page get_locked_pte() would be simpler but it's
not available to modules.
7e6bee6281