af_vsock: separate receive data loop

Some code in receive data loop could be shared between SEQPACKET
and STREAM sockets, while another part is type specific, so move STREAM
specific data receive logic to '__vsock_stream_recvmsg()' dedicated
function, while checks, that will be same for both STREAM and SEQPACKET
sockets, stays in 'vsock_connectible_recvmsg()'.

Signed-off-by: Arseny Krasnov <arseny.krasnov@kaspersky.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>