Fix: relayd metadata size

Trying to see better how the metadata size was handled, I ended up doing
a couple of modifications to the size value sent by the sender: given
that this header contains a size value that should cover both the
metadata packet header size, and the following payload size (this is how
kernel splice sent behaves), I modified kernel/UST mmap to do the same.

I therefore changed the recently updated recv_metadata payload size
calculation to match the encoded size, which should hopefully work for
all kernel splice/mmap and UST mmap cases.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>