Make the description of @policy to @p in nla_policy_len()
to clear the below warnings:

lib/nlattr.c:660: warning: Function parameter or member 'p' not described in 'nla_policy_len'
lib/nlattr.c:660: warning: Excess function parameter 'policy' description in 'nla_policy_len'

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2736Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Link: https://lore.kernel.org/r/20221107062623.6709-1-yang.lee@linux.alibaba.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>