arm64: Add comment for stack_info::kr_cur

We added stack_info::kr_cur in commit:

  cd9bc2c9258816dc ("arm64: Recover kretprobe modified return address in stacktrace")

... but didn't add anything in the corresponding comment block.

For consistency, add a corresponding comment.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviwed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20211129142849.3056714-3-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/stacktrace.h

@@ -47,6 +47,10 @@ struct stack_info {
  * @prev_type:   The type of stack this frame record was on, or a synthetic
  *               value of STACK_TYPE_UNKNOWN. This is used to detect a
  *               transition from one stack to another.
+ *
+ * @kr_cur:      When KRETPROBES is selected, holds the kretprobe instance
+ *               associated with the most recently encountered replacement lr
+ *               value.
  */
 struct stackframe {
 	unsigned long fp;