ipv6: ioam: block BH from ioam6_output()

As explained in commit 13788174 ("tipc: block BH
before using dst_cache"), net/core/dst_cache.c
helpers need to be called with BH disabled.

Disabling preemption in ioam6_output() is not good enough,
because ioam6_output() is called from process context,
lwtunnel_output() only uses rcu_read_lock().

We might be interrupted by a softirq, re-enter ioam6_output()
and corrupt dst_cache data structures.

Fix the race by using local_bh_disable() instead of
preempt_disable().

Fixes: 8cb3bf8b

 ("ipv6: ioam: Add support for the ip6ip6 encapsulation")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Justin Iurman <justin.iurman@uliege.be>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240531132636.2637995-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>