perf trace: Make "--call-graph" affect just "raw_syscalls:sys_exit"

perf trace: Make "--call-graph" affect just "raw_syscalls:sys_exit"

We don't need the callchains at the syscall enter tracepoint, just when
finishing it at syscall exit, so reduce the overhead by asking for
callchains just at syscall exit.

Suggested-by: Milian Wolff <milian.wolff@kdab.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-fja1ods5vqpg42mdz09xcz3r@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>