itimer: Serialize access to the p_itimers array

Fix the following race between itimer_proc_continue() and process exit.

itimer_proc_continue() may be called via realitexpire(), the real
interval timer. Note that exit1() drains this timer _after_ draining
and freeing itimers. Moreover, itimers_exit() is called without the
process lock held; it only acquires the proc lock when deleting
individual itimers, so once they are drained we free p->p_itimers
without any synchronization. Thus, itimer_proc_continue() may load a
non-NULL p->p_itimers array and iterate over it after it has been freed.

Fix the problem by using the process lock when clearing p->p_itimers, to
synchronize with itimer_proc_continue(). Formally, accesses to this
field should be protected by the process lock anyway, and since the
array is allocated lazily this will not incur any overhead in the common
case.

Reported by: syzbot+c40aa8bf54fe333fc50b@syzkaller.appspotmail.com
Reported by: syzbot+929be2f32503bbc3844f@syzkaller.appspotmail.com
Reviewed by: kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 3138392a46a4a8ecfb8e36e9970e88bbae9caed3)