rsocket: Modify when control messages are available

rsocket: Modify when control messages are available

Rsockets currently tracks how many control messages (i.e.
entries in the send queue) that are available using a
single ctrl_avail counter.  Seems simple enough.

However, control messages currently require the use of
inline data.  In order to support control messages that
do not use inline data, we need to associate each
control message with a specific data buffer.  This will
become easier to manage if we modify how we track when
control messages are available.

We replace the single ctrl_avail counter with two new
counters.  The new counters conceptually treat control
messages as if each message had its own sequence number.
The sequence number will then be able to correspond to
a specific data buffer in a follow up patch.

ctrl_seqno will be used to indicate the current control
message being sent.  ctrl_max_seqno will track the
highest control message that may be sent.

A side effect of this change is that we will be able to
see how many control messages have been sent.