powerpc/32s: In add_hash_page(), calculate VSID later

VSID is only for create_hpte(). When _PAGE_HASHPTE is
already set, add_hash_page() bails out without calling
create_hpte() and doesn't need the value of VSID.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3907199974c89b85a3441cf3f528751173b7649c.1606247495.git.christophe.leroy@csgroup.eu