powerpc/vdso: Don't map VDSO at a fixed address on PPC32

PPC64 removed default mapping address from VDSO in
commit 30d0b368 ("powerpc: Move 64bit VDSO to improve context
switch performance").

Do like PPC64 and let get_unmapped_area() place the VDSO mapping
at the address it wants, don't force a default address.

This allows randomisation of VDSO address.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/cba76f5a5b01fcc49415e632d92c11c1c5998cab.1660843877.git.christophe.leroy@csgroup.eu