ipv6: Fix ECMP sibling count mismatch when clearing RTF_ADDRCONF

In the Linux kernel, the following vulnerability has been resolved:

ipv6: Fix ECMP sibling count mismatch when clearing RTF_ADDRCONF

syzbot reported a kernel BUG in fib6_add_rt2node() when adding an IPv6
route. [0]

Commit f72514b3c569 ("ipv6: clear RA flags when adding a static
route") introduced logic to clear RTF_ADDRCONF from existing routes
when a static route with the same nexthop is added. However, this
causes a problem when the existing route has a gateway.

When RTF_ADDRCONF is cleared from a route that has a gateway, that
route becomes eligible for ECMP, i.e. rt6_qualify_for_ecmp() returns
true. The issue is that this route was never added to the
fib6_siblings list.

This leads to a mismatch between the following counts:

- The sibling count computed by iterating fib6_next chain, which
includes the newly ECMP-eligible route

- The actual siblings in fib6_siblings list, which does not include
that route

When a subsequent ECMP route is added, fib6_add_rt2node() hits
BUG_ON(sibling->fib6_nsiblings != rt->fib6_nsiblings) because the
counts don't match.

Fix this by only clearing RTF_ADDRCONF when the existing route does
not have a gateway. Routes without a gateway cannot qualify for ECMP
anyway (rt6_qualify_for_ecmp() requires fib_nh_gw_family), so clearing
RTF_ADDRCONF on them is safe and matches the original intent of the
commit.

[0]:
kernel BUG at net/ipv6/ip6_fib.c:1217!
Oops: invalid opcode: 0000 [#1] SMP KASAN PTI
CPU: 0 UID: 0 PID: 6010 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:fib6_add_rt2node+0x3433/0x3470 net/ipv6/ip6_fib.c:1217
[...]
Call Trace:
<TASK>
fib6_add+0x8da/0x18a0 net/ipv6/ip6_fib.c:1532
__ip6_ins_rt net/ipv6/route.c:1351 [inline]
ip6_route_add+0xde/0x1b0 net/ipv6/route.c:3946
ipv6_route_ioctl+0x35c/0x480 net/ipv6/route.c:4571
inet6_ioctl+0x219/0x280 net/ipv6/af_inet6.c:577
sock_do_ioctl+0xdc/0x300 net/socket.c:1245
sock_ioctl+0x576/0x790 net/socket.c:1366
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f