unwind_user/deferred: Add unwind_user_faultable()

Add a new API to retrieve a user space callstack called
unwind_user_faultable(). The difference between this user space stack
tracer from the current user space stack tracer is that this must be
called from faultable context as it may use routines to access user space
data that needs to be faulted in.

It can be safely called from entering or exiting a system call as the code
can still be faulted in there.

This code is based on work by Josh Poimboeuf's deferred unwinding code:

Link: https://lore.kernel.org/all/6052e8487746603bdb29b65f4033e739092d9925.1737511963.git.jpoimboe@kernel.org/

Cc: Masami Hiramatsu <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Indu Bhagat <[email protected]>
Cc: "Jose E. Marchesi" <[email protected]>
Cc: Beau Belgrave <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Florian Weimer <[email protected]>
Cc: Sam James <[email protected]>
Link: https://lore.kernel.org/[email protected]
Reviewed-by: Jens Remus <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

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