powerpc/mm: Avoid unnecessary test and reduce code size

no_selective_tlbil hence the use of either steal_all_contexts()
or steal_context_up() depends on the subarch, it won't change
during run. Only the 8xx uses steal_all_contexts and CONFIG_PPC_8xx
is exclusive of other processors.

This patch replaces the test of no_selective_tlbil global var by
a test of CONFIG_PPC_8xx selection. It avoids the test and
removes unnecessary code.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>