powerpc/vdso: Remove get_page() in vdso_pagelist initialization

Partly copied from commit 16fb1a9b ("arm64: vdso: clean up
vdso_pagelist initialization").

No need to get_page() the vdso text/data - these are part of the
kernel image.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9d14540bd10832b6c9519d74fb5728fdc4974b36.1601197618.git.christophe.leroy@csgroup.eu