LSM: Update comment on security_sock_rcv_skb

It is not permitted to do sleeping operation inside security_sock_rcv_skb().

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Serge Hallyn <serue@us.ibm.com>

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Signed-off-by: James Morris <jmorris@namei.org>