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: 4792adba ("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>