ipv6: Add sanity checks on ipv6_devconf.rpl_seg_enabled

In ipv6_rpl_srh_rcv() we use min(net->ipv6.devconf_all->rpl_seg_enabled,
idev->cnf.rpl_seg_enabled) is intended to return 0 when either value is
zero, but if one of the values is negative it will in fact return non-zero.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Link: https://patch.msgid.link/20250901123726.1972881-3-yuehaibing@huawei.com


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