UPSTREAM: arm64: vdso: add __init section marker to alloc_vectors_page

(cherry pick from commit 1aed28f9)

It is not needed after booting, this patch moves the alloc_vectors_page
function to the __init section.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Bug: 20045882
Bug: 19198045
Change-Id: I1d7b89f9e20890b18e77c8752f1eef33d721ea81