powerpc/vdso64: Switch from __get_datapage() to get_datapage inline macro

On the same way as already done on PPC32, drop __get_datapage()
function and use get_datapage inline macro instead.

See commit ec0895f0 ("powerpc/vdso32: inline __get_datapage()")

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e13d95312e0b9792556b19b4bb8955cc1ff19fc7.1588079622.git.christophe.leroy@c-s.fr