powerpc/perf_counter: Fix vdso detection

perf_counter uses arch_vma_name() to detect a vdso region which in turn uses
current->mm->context.vdso_base. We need to initialise this before doing
the mmap or else we fail to detect the vdso.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>