vsock: Update msg_count on read_skb()

Dequeuing via vsock_transport::read_skb() left msg_count outdated, which
then confused SOCK_SEQPACKET recv(). Decrease the counter.

Fixes: 634f1a71 ("vsock: support sockmap")
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20241013-vsock-fixes-for-redir-v2-3-d6577bbfe742@rbox.co