kernel/sys.c: only take tasklist_lock for get/setpriority(PRIO_PGRP)

kernel/sys.c: only take tasklist_lock for get/setpriority(PRIO_PGRP)

PRIO_PGRP needs the tasklist_lock mainly to serialize vs setpgid(2), to
protect against any concurrent change_pid(PIDTYPE_PGID) that can move the
task from one hlist to another while iterating.

However, the remaining can only rely only on RCU:

PRIO_PROCESS only does the task lookup and never iterates over tasklist
and we already have an rcu-aware stable pointer.

PRIO_USER is already racy vs setuid(2) so with creds being rcu protected,
we can end up seeing stale data.  When removing the tasklist_lock there
can be a race with (i) fork but this is benign as the child's nice is
inherited and the new task is not observable by the user yet either, hence
the return semantics do not differ.  And (ii) a race with exit, which is a
small window and can cause us to miss a task which was removed from the
list and it had the highest nice.

Similarly change the buggy do_each_thread/while_each_thread combo in
PRIO_USER for the rcu-safe for_each_process_thread flavor, which doesn't
make use of next_thread/p->thread_group.

Link: https://lkml.kernel.org/r/20211210182250.43734-1-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>