Using sk_dst_lock from softirq context is not supported right now.

Instead of adding BH protection everywhere,
udp_sk_rx_dst_set() can instead use xchg(), as suggested
by David.
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Fixes: 97502231 ("udp: ipv4: must add synchronization in udp_sk_rx_dst_set()")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>