fs/proc: do_task_stat: use sig->stats_lock to gather the threads/children stats

In the Linux kernel, the following vulnerability has been resolved:

fs/proc: do_task_stat: use sig->stats_lock to gather the threads/children stats

lock_task_sighand() can trigger a hard lockup. If NR_CPUS threads call

do_task_stat() at the same time and the process has NR_THREADS, it will

spin with irqs disabled O(NR_CPUS * NR_THREADS) time.

Change do_task_stat() to use sig->stats_lock to gather the statistics

outside of ->siglock protected section, in the likely case this code will

run lockless.