powerpc/nohash: Deduplicate ptep_set_wrprotect() and ptep_get_and_clear()

ptep_set_wrprotect() and ptep_get_and_clear are identical for
nohash/32 and nohash/64.

Make them common.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/ffe46edecdabce915e2d1a4b79a3b2ab770f2248.1695659959.git.christophe.leroy@csgroup.eu