unwind_user/deferred: Add unwind_user_faultable()

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 <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: 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/20250729182405.147896868@kernel.org
Reviewed-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>