ipv6: fix data race in fib6_metric_set() using cmpxchg

ipv6: fix data race in fib6_metric_set() using cmpxchg

fib6_metric_set() may be called concurrently from softirq context without
holding the FIB table lock. A typical path is:

  ndisc_router_discovery()
    spin_unlock_bh(&table->tb6_lock)        <- lock released
    fib6_metric_set(rt, RTAX_HOPLIMIT, ...) <- lockless call

When two CPUs process Router Advertisement packets for the same router
simultaneously, they can both arrive at fib6_metric_set() with the same
fib6_info pointer whose fib6_metrics still points to dst_default_metrics.

  if (f6i->fib6_metrics == &dst_default_metrics) {   /* both CPUs: true */
      struct dst_metrics *p = kzalloc_obj(*p, GFP_ATOMIC);
      refcount_set(&p->refcnt, 1);
      f6i->fib6_metrics = p;   /* CPU1 overwrites CPU0's p -> p0 leaked */
  }

The dst_metrics allocated by the losing CPU has refcnt=1 but no pointer
to it anywhere in memory, producing a kmemleak report:

  unreferenced object 0xff1100025aca1400 (size 96):
    comm "softirq", pid 0, jiffies 4299271239
    backtrace:
      kmalloc_trace+0x28a/0x380
      fib6_metric_set+0xcd/0x180
      ndisc_router_discovery+0x12dc/0x24b0
      icmpv6_rcv+0xc16/0x1360

Fix this by:
 - Set val for p->metrics before published via cmpxchg() so the metrics
   value is ready before the pointer becomes visible to other CPUs.
 - Replace the plain pointer store with cmpxchg() and free the allocation
   safely when competition failed.
 - Add READ_ONCE()/WRITE_ONCE() for metrics[] setting in the non-default
   metrics path to prevent compiler-based data races.

Fixes: d4ead6b34b67 ("net/ipv6: move metrics from dst to rt6_info")
Reported-by: Fei Liu <feliu@redhat.com>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260331-b4-fib6_metric_set-kmemleak-v3-1-88d27f4d8825@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>