perf script: Fix named threads support

perf script: Fix named threads support

Commit bd7c2d0 broke named thread support in perf-script. The thread
struct in al is the main thread for a multithreaded process. The thread
struct used for analysis (e.g., dumping events) should be the specific
thread for the sample.

Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Feng Tang <feng.tang@intel.com>
Link: http://lkml.kernel.org/r/1374185175-28272-1-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>