powerpc/mm/hash: Compute the segment size correctly for ISA 3.0

PowerISA 3.0 encodes the segment size in the second half of hash page
table entry. Update hpte_decode() accordingly.

Fixes: 50de596d ("powerpc/mm/hash: Add support for Power9 Hash")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>