hsr_forward_skb() a skb and keeps information in an on-stack
hsr_frame_info. hsr_get_node() assigns hsr_frame_info::node_src which is
from a RCU list. This pointer is used later in hsr_forward_do().
I don't see a reason why this pointer can't vanish midway since there is
no guarantee that hsr_forward_skb() is invoked from an RCU read section.
Use rcu_read_lock() to protect hsr_frame_info::node_src from its
assignment until it is no longer used.
Fixes: f266a683a4804 ("net/hsr: Better frame dispatch") Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Jakub Kicinski <kuba@kernel.org>