ANDROID: uid_sys_stats: Use a single work for deferred updates

uid_sys_stats tries to acquire a lock when any task exits to do some
bookkeeping in common data structure. If the lock is contended, it
allocates and schedules a work to do the work later to avoid task exit
latency.

In a stress test which creates many tasks exiting, the workqueue can be
overwhelmed by the number of works being scheduled and allocates more
worker threads to handle queue. The growth of the number of threads is
effectively unbounded and can exhaust the process table. This causes
denial of service to userspace trying to fork().

Instead of allocating a new work each, create a linked list of the
update stats deferred work and have a single work to drain the linked
list. The linked list is implemented using an atomic_long_t.

Bug: 294468796
Fixes: 5586278c ("ANDROID: uid_sys_stats: defer process_notifier work if uid_lock is contended")
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
(cherry picked from https://android-review.googlesource.com/q/commit:8e86825eecfaaa582ab51a0924b469d2d2adc743)
Merged-In: I15f20f4f69ea66a452bdf815c4ef3a0da3edfd36
Change-Id: I15f20f4f69ea66a452bdf815c4ef3a0da3edfd36