tcpstates: forget sockets when connection is closed
tcpstates: forget sockets when connection is closed

The adress of struct sock, which are used as the map key, are often
reused. When it happens random duration appears in the MS field for
new connections (CLOSE->SYN_SENT and LISTEN->SYN_RECV
transitions). Let's forget about the socket when the connection is
closed.

iovisor/bcc,brendangregg/bcc,iovisor/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,iovisor/bcc,brendangregg/bcc,brendangregg/bcc,brendangregg/bcc