greybus: simplify pending operations tracking

greybus: simplify pending operations tracking

Greg raised the alarm when I first put in the red-black tree for
tracking pending operations.  The reality as that we're not likely
to have that many operations in flight at any one time, so the
complexity of the red-black tree is most likely unwarranted.  I
already

This pulls out the red-black tree and uses a simple list instead.  A
connection maintains two lists of operations.  An operation starts
on its connection's operations list.  It is moved to the pending
list when its request message is sent.  And it is moved back to
the operations list when the response message arrives.  It is
removed from whatever list it's in when the operation is destroyed.
We reuse the single operation->links field for both lists.

Only outgoing requests are ever "pending."  Incoming requests are
transient--we receive them, process them, send the response, and
then we're done.

Change a few function names so it's clear we're working with the
pending list.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>