perf symbols: Fill in pgoff in mmap synthesized events

When we synthesize mmap events we need to fill in the pgoff field.

I wasn't able to test this completely since I couldn't find an
executable region with a non 0 offset. We will see it when we start
doing data profiling.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: David Miller <davem@davemloft.net>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <20100403115331.GK5594@kryten>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>