Move the uffd*() routines to their natural home. Note that
ksm_functional_tests.c also depend, intentionally (due to a recent
commit [1]), upon uffd-common.[ch].
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>