posix-timers: Use sighand lock instead of tasklist_lock for task clock sample
posix-timers: Use sighand lock instead of tasklist_lock for task clock sample

There is no need for the tasklist_lock just to take a process
wide clock sample.

All we need is to get a coherent sample that doesn't race with
exit() and exec():

* exit() may be concurrently reaping a task and flushing its time

* sighand is unstable under exit() and exec(), and the latter also
  result in group leader that can change

To protect against these, locking the target's sighand is enough.

Signed-off-by: Frederic Weisbecker <e8a1bf9163cb25e93cfd6540f223b3872ea7ee55@gmail.com>
Cc: Thomas Gleixner <00e4cf8f46a57000a44449bf9dd8cbbcc209fd2a@linutronix.de>
Cc: Ingo Molnar <9dbbbf0688fedc85ad4da37637f1a64b8c718ee2@kernel.org>
Cc: Peter Zijlstra <3fddac958924aef220f202ca567388ddab3f14a8@infradead.org>
Cc: Oleg Nesterov <20b70f0af00562e63758b9ee42012ecc96c58590@redhat.com>
Cc: Kosaki Motohiro <70a1d3ef3e17a2bb0f09a1b2e6c86f607ed1d6d9@jp.fujitsu.com>
Cc: Andrew Morton <5c1e68b099950c134891f0b6e179498a8ebe9cf9@linux-foundation.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs