cpuview: fix possible use-after-free in find_proc_stat_node

cpuview: fix possible use-after-free in find_proc_stat_node

Our current lock design uses 2 sync primitives.
First (pthread_rwlock) protects hash table buckets.
Second (pthread_mutex) protects each struct cg_proc_stat
from concurrent modification. But the problem is that function
find_proc_stat_node() can return a pointer to the node
(struct cg_proc_stat) which can be freed by prune_proc_stat_history()
call *before* we take pthread_mutex. Moreover, we perform
memory release of (struct cg_proc_stat) in prune_proc_stat_list()
without any protection like refcounter or mutex on (struct cg_proc_stat).

An attempt to guess what happens in:
https://github.com/lxc/lxcfs/issues/565
https://discuss.linuxcontainers.org/t/number-of-cpus-reported-by-proc-stat-fluctuates-causing-issues/15780/14

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>