exit: combine work under lock in synchronize_group_exit() and coredump_task_exit()

This reduces single-threaded overhead as it avoids one lock+irq trip on
exit.

It also improves scalability of spawning and killing threads within one
process (just shy of 5% when doing it on 24 cores on my test jig).

Both routines are moved below kcov and kmsan exit, which should be
harmless.

Link: https://lkml.kernel.org/r/20250319195436.1864415-1-mjguzik@gmail.com
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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