x86: Set context.vdso before installing the mapping

In order to make arch_vma_name() work from inside
install_special_mapping() we need to set the context.vdso
before calling it.

( This is needed for performance counters to be able to track
  this special executable area. )

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>