This patch tries to fix it by holding clcsock_release_lock and
checking whether clcsock has already been released before access.
In case that a crash of the same reason happens in smc_getsockopt()
or smc_switch_to_fallback(), this patch also checkes smc->clcsock
in them too. And the caller of smc_switch_to_fallback() will identify
whether fallback succeeds according to the return value.
