powerpc/mm/nvdimm: Add an informative message if we fail to allocate altmap block

Allocation from altmap area can fail based on vmemmap page size used.
Add kernel info message to indicate the failure. That allows the user
to identify whether they are really using persistent memory reserved
space for per-page metadata.

The message looks like:
  [  136.587212] altmap block allocation failed, falling back to system memory

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Reviewed-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>