uml: add virt_to_pte

Turn um_virt_to_phys into virt_to_pte, cleaning up a horrid interface.

It's also made non-static and declared in pgtable.h because it'll be
needed when the stubs get a vma.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>