bpf: sockmap, hash table is RCU so readers do not need locks

This removes locking from readers of RCU hash table. Its not
necessary.

Fixes: 81110384

 ("bpf: sockmap, add hash map support")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>