The new logging in 2dd71e64ac is spammy at shutdown:
Receive func ReceiveIPv6 exiting with error: *net.OpError, read udp [::]:38869: raw-read udp6 [::]:38869: use of closed network connection
Receive func ReceiveIPv4 exiting with error: *net.OpError, read udp 0.0.0.0:36123: raw-read udp4 0.0.0.0:36123: use of closed network connection
Skip it if we're in the process of shutting down.
Updates #10976
Change-Id: I4f6d1c68465557eb9ffe335d43d740e499ba9786
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>