rx: dec rx_nWaiting on clearing RX_CALL_WAIT_PROC

rx: dec rx_nWaiting on clearing RX_CALL_WAIT_PROC

Currently, a couple of callers (rxi_ResetCall, and
rxi_AttachServerProc) will decrement rx_nWaiting only if
RX_CALL_WAIT_PROC is set for a call, and the call is on a queue
(presumably rx_incomingCallQueue). This can cause an imbalance in
rx_nWaiting if these code paths are reached when, in another thread,
rx_GetCall has removed the call from its queue, but it has not yet
cleared RX_CALL_WAIT_PROC (this can happen while it is waiting for
call->lock). In this situation, rx_GetCall will remove the call from
its queue, wait, and e.g. rxi_ResetCall will clear RX_CALL_WAIT_PROC;
neither will decrement rx_nWaiting.

This is possible if a new call is started on a call channel with an
extant call that is waiting for a thread; we will rxi_ResetCall in
rxi_ReceivePacket, but rx_GetCall may be running at the same time.
This race may also be possible via rxi_AttachServerProc via
rxi_UpdatePeerReach -> TryAttach -> rxi_AttachServerProc while
rx_GetCall is running, but I'm not sure.

To avoid this, decrement rx_nWaiting based on RX_CALL_WAIT_PROC alone,
regardless of whether or not the call is on a queue. This mirrors the
incrementing rx_nWaiting behavior, where rx_nWaiting is only
incremented if RX_CALL_WAIT_PROC is unset for a call, so this should
guarantee that rx_nWaiting does not become unbalanced.

Change-Id: I7dba4ba5f7cc33270c2d0f486b850fc0391927d1
Reviewed-on: http://gerrit.openafs.org/6986
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>