arm64: Add comment for stack_info::kr_cur

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>