net: tcp: Fix flag goof in net_tcp_alloc()

My previous race cleanup missed the fact that the memset would end up
clearing the IN_USE flag!  So we were always allocating the same
struct net_tcp (which, because the typical use is to listen on one
socket and then transfer on an accepted socket, almost kinda worked
for a lot of stuff, making this hard to find).

Change-Id: I8ca0c7f835ebd72271df10d03004f38f8b8efbd5
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>