VSOCK: define VSOCK_SS_LISTEN once only

The SS_LISTEN socket state is defined by both af_vsock.c and
vmci_transport.c.  This is risky since the value could be changed in one
file and the other would be out of sync.

Rename from SS_LISTEN to VSOCK_SS_LISTEN since the constant is not part
of enum socket_state (SS_CONNECTED, ...).  This way it is clear that the
constant is vsock-specific.

The big text reflow in af_vsock.c was necessary to keep to the maximum
line length.  Text is unchanged except for s/SS_LISTEN/VSOCK_SS_LISTEN/.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>