pidns: fix NULL dereference in __task_pid_nr_ns()

I got a crash during a "perf top" session that was caused by a race in
__task_pid_nr_ns() :

pid_nr_ns() was inlined, but apparently compiler chose to read
task->pids[type].pid twice, and the pid->level dereference crashed
because we got a NULL pointer at the second read :

    if (pid && ns->level <= pid->level) { // CRASH

Just use RCU API properly to solve this race, and not worry about "perf
top" crashing hosts :(

get_task_pid() can benefit from same fix.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>