vsock/test: track bytes in sk_buff merging test for SOCK_SEQPACKET

The test was a bit complicated to read.
Added variables to keep track of the bytes read and to be read
in each step. Also some comments.

The test is unchanged.

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