ipv6: annotate data-races around devconf->rpl_seg_enabled

devconf->rpl_seg_enabled can be changed concurrently from
/proc/sys/net/ipv6/conf, annotate lockless reads on it.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Link: https://patch.msgid.link/20250901123726.1972881-2-yuehaibing@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>