powerpc/vdso: Move vdso datapage up front

Move the vdso datapage in front of the VDSO area,
before vdso test.

This will allow to remove the __kernel_datapage_offset symbol
and simplify __get_datapage() in following patches.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b68c99b6e8ee0b1d99bfa4c7e34c359fc1bc1000.1601197618.git.christophe.leroy@csgroup.eu