Prevent a race condition. As pf_send_tcp() - called for expired synproxy
states - has to drop the lock when calling back to ip_output(), the state
purge timeout might run and gc the state. This results in a rb-tree
inconsistency. With this change we flag expiring states while holding the
lock and back off if the flag is already set.

Reported by: glebius
MFC after: 2 weeks