VSOCK: Fix missing msg_namelen update in vsock_stream_recvmsg()

The code misses to update the msg_namelen member to 0 and therefore
makes net/socket.c leak the local, uninitialized sockaddr_storage
variable to userland -- 128 bytes of kernel stack memory.

Cc: Andy King <acking@vmware.com>
Cc: Dmitry Torokhov <dtor@vmware.com>
Cc: George Zhang <georgezhang@vmware.com>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>