kcm: return immediately after copy_from_user() failure

[ Upstream commit a80db69e ]

There is no reason to continue after a copy_from_user()
failure.

Fixes: ab7ac4eb ("kcm: Kernel Connection Multiplexor module")
Cc: Tom Herbert <tom@herbertland.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>