util: kernel: Check whether adding cpu e/u/stime for TGID taskstat

util: kernel: Check whether adding cpu e/u/stime for TGID taskstat

There are difference with information on TGID taskstat among kernel
version, because e/u/stime for TGID taskstat is only summarized after
commit in mainline linux 8c733420bdd5 ("taskstats: add e/u/stime for TGID command")
is applied.
So, now check TGID taskstat has zero value for e/u/stime at the
initialization, and, if so, those time is calculated by adding values
in PID taskstats.

Change-Id: I8916e754465102a6643aefb8937e763fc5d1ee7a
Signed-off-by: Dongwoo Lee <dwoo08.lee@samsung.com>