powerpc/32s: Inline tlb_flush()

On book3s/32, tlb_flush() does nothing when the CPU has a hash table,
it calls _tlbia() otherwise.

Inline it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ebc933d1c530a19ef3cf7983f6ae94814f6e92ac.1603348103.git.christophe.leroy@csgroup.eu