Rx: restore thread safety to rx_NewCall

Rx: restore thread safety to rx_NewCall

Thread safety in rx_NewCall requires that only one thread be
actively allocating or recycling a call at a time.  Since we are
no longer holding the conn_call_lock across the entire transaction
we need to have another synchronization mechanism.  Add a new
rx_connection flag, RX_CONN_MAKECALL_ACTIVE, which when set indicates
that a thread is actively obtaining a call.  If any other threads
see this flag set, they will wait until being signalled that the
thread has completed its activity.

In addition, because the call->lock may be dropped when processing
rxi_ResetCall(), we must hold a reference to the call once we
begin using it.  Otherwise, the call may be garbage collected
behind our back.

Change-Id: Ie97757f812d7043203ffcaf399400789cda39da1
Reviewed-on: http://gerrit.openafs.org/1755
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>