unwind deferred: Use bitmask to determine which callbacks to call

In order to know which registered callback requested a stacktrace for when
the task goes back to user space, add a bitmask to keep track of all
registered tracers. The bitmask is the size of long, which means that on a
32 bit machine, it can have at most 32 registered tracers, and on 64 bit,
it can have at most 64 registered tracers. This should not be an issue as
there should not be more than 10 (unless BPF can abuse this?).

When a tracer registers with unwind_deferred_init() it will get a bit
number assigned to it. When a tracer requests a stacktrace, it will have
its bit set within the task_struct. When the task returns back to user
space, it will call the callbacks for all the registered tracers where
their bits are set in the task's mask.

When a tracer is removed by the unwind_deferred_cancel() all current tasks
will clear the associated bit, just in case another tracer gets registered
immediately afterward and then gets their callback called unexpectedly.

To prevent live locks from happening if an event that happens between the
task_work and when the task goes back to user space, triggers the deferred
unwind, have the unwind_mask get cleared on exit to user space and not
after the callback is made.

Move the pending bit from a value on the task_struct to bit zero of the
unwind_mask (saves space on the task_struct). This will allow modifying
the pending bit along with the work bits atomically.

Instead of clearing a work's bit after its callback is called, it is
delayed until exit. If the work is requested again, the task_work is not
queued again and the request will be notified that the task has already been
called by returning a positive number (the same as if it was already
pending).

The pending bit is cleared before calling the callback functions but the
current work bits remain. If one of the called works registers again, it
will not trigger a task_work if its bit is still present in the task's
unwind_mask.

If a new work requests a deferred unwind, then it will set both the
pending bit and its own bit. Note this will also cause any work that was
previously queued and had their callback already executed to be executed
again. Future work will remove these spurious callbacks.

The use of atomic_long bit operations were suggested by Peter Zijlstra:
Link: https://lore.kernel.org/all/20250715102912.GQ1613200@noisy.programming.kicks-ass.net/
The unwind_mask could not be converted to atomic_long_t do to atomic_long
not having all the bit operations needed by unwind_mask. Instead it
follows other use cases in the kernel and just typecasts the unwind_mask
to atomic_long_t when using the two atomic_long functions.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Indu Bhagat <indu.bhagat@oracle.com>
Cc: "Jose E. Marchesi" <jemarch@gnu.org>
Cc: Beau Belgrave <beaub@linux.microsoft.com>
Cc: Jens Remus <jremus@linux.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Sam James <sam@gentoo.org>
Link: https://lore.kernel.org/20250729182405.822789300@kernel.org
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

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