powerpc/nohash: Refactor checking of no-change in pte_update()

On nohash/64, a few callers of pte_update() check if there is
really a change in order to avoid an unnecessary write.

Refactor that inside pte_update().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/076563e611c2b51036686a8d378bfd5ef1726341.1695659959.git.christophe.leroy@csgroup.eu