powerpc/vdso: Simplify arch_setup_additional_pages() exit

To simplify arch_setup_additional_pages() exit, rename
it __arch_setup_additional_pages() and create a caller
arch_setup_additional_pages() which does the locking.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/603c1d039d3f928ee95e547fcd2219fcf4c3b514.1601197618.git.christophe.leroy@csgroup.eu