[SOCK]: Adds a rcu_dereference() in sk_filter

It seems commit fda9ef5d679b07c9d9097aaf6ef7f069d794a8f9 introduced a RCU
protection for sk_filter(), without a rcu_dereference()

Either we need a rcu_dereference(), either a comment should explain why we
dont need it. I vote for the former.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>