tracing: Make trace_user_fault_read() exposed to rest of tracing

The write to the trace_marker file is a critical section where it cannot
take locks nor allocate memory. To read from user space, it allocates a per
CPU buffer when the trace_marker file is opened, and then when the write
system call is performed, it uses the following method to read from user
space:

	preempt_disable();
	buffer = per_cpu_ptr(cpu_buffers, cpu);
	do {
		cnt = nr_context_switches_cpu();
		migrate_disable();
		preempt_enable();
		ret = copy_from_user(buffer, ptr, len);
		preempt_disable();
		migrate_enable();
	} while (!ret && cnt != nr_context_switches_cpu());
	if (!ret)
		ring_buffer_write(buffer);
	preempt_enable();

It records the number of context switches for the current CPU, enables
preemption, copies from user space, disable preemption and then checks if
the number of context switches changed. If it did not, then the buffer is
valid, otherwise the buffer may have been corrupted and the read from user
space must be tried again.

The system call trace events are now faultable and have the same
restrictions as the trace_marker write. For system calls to read the user
space buffer (for example to read the file of the openat system call), it
needs the same logic. Instead of copying the code over to the system call
trace events, make the code generic to allow the system call trace events to
use the same code. The following API is added internally to the tracing sub
system (these are only exposed within the tracing subsystem and not to be
used outside of it):

  trace_user_fault_init() - initializes a trace_user_buf_info descriptor
       that will allocate the per CPU buffers to copy from user space into.

  trace_user_fault_destroy() - used to free the allocations made by
       trace_user_fault_init().

  trace_user_fault_get() - update the ref count of the info descriptor to
       allow more than one user to use the same descriptor.

  trace_user_fault_put() - decrement the ref count.

  trace_user_fault_read() - performs the above action to read user space
      into the per CPU buffer. The preempt_disable() is expected before
      calling this function and preemption must remain disabled while the
      buffer returned is in use.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Takaya Saeki <takayas@google.com>
Cc: Tom Zanussi <zanussi@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ian Rogers <irogers@google.com>
Cc: Douglas Raillard <douglas.raillard@arm.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Link: https://lore.kernel.org/20251028231147.096570057@kernel.org
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

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