kprobes: Add missing kerneldoc for __get_insn_slot

Add kerneldoc for '__get_insn_slot' function to fix W=1 warnings:

  kernel/kprobes.c:141 function parameter 'c' not described in '__get_insn_slot'

Link: https://lore.kernel.org/all/20250704143817707TOCcfTRWsO5OAbQ2eYoU9@zte.com.cn/



Signed-off-by: Peng Jiang <jiang.peng9@zte.com.cn>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>