powerpc/mm: Fix check of multiple 16G pages from device tree

commit 23493c121912a39f0262e0dbeb236e1d39efa4d5 upstream.

The offset of hugepage block will not be 16G, if the expected
page is more than one. Calculate the totol size instead of the
hardcode value.

Fixes: 4792adbac9eb ("powerpc: Don't use a 16G page if beyond mem= limits")
Signed-off-by: Rui Teng <rui.teng@linux.vnet.ibm.com>
Tested-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>