powerpc/mm: Use pte_user() instead of open coding

We have a common declaration in pte-common.h Add a book3s specific one
and switch to pte_user() in callchain.c. In a subsequent patch we will
switch _PAGE_USER to _PAGE_PRIVILEGED in the book3s version only.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>