ftrace: Make ftrace_graph_ent depth field signed

The code has integrity checks to make sure that depth never goes below
zero. But the depth field has recently been converted to unsigned long
from "int" (for alignment reasons). As unsigned long can never be less
than zero, the integrity checks no longer work.

Convert depth to long from unsigned long to allow the integrity checks to
work again.

Cc: stable@vger.kernel.org
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: pengdonglin <pengdonglin@xiaomi.com>
Link: https://patch.msgid.link/20260102143148.251c2e16@gandalf.local.home
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/aS6kGi0maWBl-MjZ@stanley.mountain/
Fixes: f83ac75 ("function_graph: Enable funcgraph-args and funcgraph-retaddr to work simultaneously")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Author: rostedt

include/linux/ftrace.h

@@ -1167,7 +1167,7 @@ static inline void ftrace_init(void) { }
  */
 struct ftrace_graph_ent {
 	unsigned long func; /* Current function */
-	unsigned long depth;
+	long depth; /* signed to check for less than zero */
 } __packed;
 
 /*