perf symbols: Fix annotation of vdso

Older kernels attempt to prelink vdso to its virtual address.  To permit
annotation using objdump, the map__rip_2objdump() calculation must
result in that same address which we can infer from the start and offset
of the text section.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/1439556606-11297-1-git-send-email-adrian.hunter@intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>