perf_counter: Synthesize VDSO mmap event

perf record synthesizes mmap events for the running process.
Right now it just catches file mappings, but we can check for
the vdso symbol and add that too.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090716104817.517264409@samba.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>