exit: move and extend sched_process_exit() tracepoint

It is useful to be able to access current->mm at task exit to, say, record
a bunch of VMA information right before the task exits (e.g., for stack
symbolization reasons when dealing with short-lived processes that exit in
the middle of profiling session).  Currently, trace_sched_process_exit()
is triggered after exit_mm() which resets current->mm to NULL making this
tracepoint unsuitable for inspecting and recording task's
mm_struct-related data when tracing process lifetimes.

There is a particularly suitable place, though, right after
taskstats_exit() is called, but before we do exit_mm() and other exit_*()
resource teardowns.  taskstats performs a similar kind of accounting that
some applications do with BPF, and so co-locating them seems like a good
fit.  So that's where trace_sched_process_exit() is moved with this patch.

Also, existing trace_sched_process_exit() tracepoint is notoriously
missing `group_dead` flag that is certainly useful in practice and some of
our production applications have to work around this.  So plumb
`group_dead` through while at it, to have a richer and more complete
tracepoint.

Note that we can't use sched_process_template anymore, and so we use
TRACE_EVENT()-based tracepoint definition.  But all the field names and
order, as well as assign and output logic remain intact.  We just add one
extra field at the end in backwards-compatible way.

[andrii@kernel.org: document sched_process_exit and sched_process_template relation]
  Link: https://lkml.kernel.org/r/20250403174120.4087794-1-andrii@kernel.org
Link: https://lkml.kernel.org/r/20250402180925.90914-1-andrii@kernel.org
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Ingo Molnar <mingo@kernel.org>
Cc: Alexander Potapenko <glider@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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