powerpc: Inline setup_kup()

setup_kup() is used by both 64-bit and 32-bit code. However on 64-bit
it must not be __init, because it's used for CPU hotplug, whereas on
32-bit it should be __init because it calls setup_kuap/kuep() which
are __init.

We worked around that problem in the past by marking it __ref, see
commit 67d53f30 ("powerpc/mm: fix section mismatch for
setup_kup()").

Marking it __ref basically just omits it from section mismatch
checking, which can lead to bugs, and in fact it did, see commit
44b4c445

 ("powerpc/64s: Mark the kuap/kuep functions non __init")

We can avoid all these problems by just making it static inline.
Because all it does is call other functions, making it inline actually
shrinks the 32-bit vmlinux by ~76 bytes.

Make it __always_inline as pointed out by Christophe.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201214123011.311024-1-mpe@ellerman.id.au