bpf: sockmap add missing rcu_read_(un)lock in smap_data_ready

References to psock must be done inside RCU critical section.

Fixes: 174a79ff ("bpf: sockmap with sk redirect support")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>