vsock: rename vsock_has_data()

vsock_has_data() is used only by STREAM and SEQPACKET sockets,
so let's rename it to vsock_connectible_has_data(), using the same
nomenclature (connectible) used in other functions after the
introduction of SEQPACKET.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>