powerpc: Fix loss of vdso on fork on 32-bit

When we fork, init_new_context() improperly resets the vdso_base
of the new context to 0.  That means that the new process loses
access to the vdso for signal trampolines.

The initialization should be unnecessary anyway as the context
on a fresh mm should be 0 in the first place and binfmt_elf
will initialize that value for a newly loaded process.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>