powerpc/32s: Cleanup the mess in __set_pte_at()

__set_pte_at() handles 3 main cases with #ifdefs plus the 'percpu'
subcase which leads to code duplication.

Rewrite the function using IS_ENABLED() to minimise the total number
of cases and remove duplicated code.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/2322dd08217bccab25456fe8b189edf0e6a8b6dd.1692121353.git.christophe.leroy@csgroup.eu