sk-inet: if a source port isn't zero, a socket should be bound (v2)

Now crtools checks address, but it may be zero. For example,
a listen socket is bound to zero address to accept connections
from any addresses.

v2: add a comment about zero port

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>