powerpc/mm/dax: Fix the condition when checking if altmap vmemap can cross-boundary

Without this fix, the last subsection vmemmap can end up in memory even if
the namespace is created with -M mem and has sufficient space in the altmap
area.

Fixes: cf387d96 ("libnvdimm/altmap: Track namespace boundaries in altmap")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Tested-by: Sachin Sant &lt;sachinp@linux.ibm.com <mailto:sachinp@linux.ibm.com&gt;>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230616110826.344417-6-aneesh.kumar@linux.ibm.com