mptcp: fix syncookie build error on UP

kernel test robot says:
net/mptcp/syncookies.c: In function 'mptcp_join_cookie_init':
include/linux/kernel.h:47:38: warning: division by zero [-Wdiv-by-zero]
 #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))

I forgot that spinock_t size is 0 on UP, so ARRAY_SIZE cannot be used.

Fixes: 9466a1cc

 ("mptcp: enable JOIN requests even if cookies are in use")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>