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: 91687355

 ("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>