powerpc/vmemmap: Don't warn if we don't find a mapping vmemmap list entry

Now that we are handling vmemmap list allocation failure correctly, don't
WARN in section deactivate when we don't find a mapping vmemmap list entry.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200731113500.248306-2-aneesh.kumar@linux.ibm.com