kcm: fix a signedness in kcm_splice_read()

skb_splice_bits() returns int, kcm_splice_read() returns ssize_t,
both are signed.

We may need another patch to make them all ssize_t, but that
deserves a separated patch.

Fixes: 91687355b9 ("kcm: Splice support")
Reported-by: David Binderman <linuxdev.baldrick@gmail.com>
Cc: Tom Herbert <tom@herbertland.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/kcm/kcmsock.c

@@ -1483,7 +1483,7 @@ static ssize_t kcm_splice_read(struct socket *sock, loff_t *ppos,
 	long timeo;
 	struct kcm_rx_msg *rxm;
 	int err = 0;
-	size_t copied;
+	ssize_t copied;
 	struct sk_buff *skb;
 
 	/* Only support splice for SOCKSEQPACKET */