rx: Reduce dependence on call->lock

rx: Reduce dependence on call->lock

This patch reduces our dependence on call->lock, by allowing more
of the reader thread to run lock free.  Doing so requires that
call->mode only be set by the reader thread.  As a result, call->mode
can only be set to RX_CALL_ERROR by rxi_CallError().  The mode is
set to RX_CALL_ERROR by the reader thread immediately after regaining
the call->lock when it has been dropped.

Change-Id: Ie9541d8beac2d428526f8b2b4cc0004219e820be
Reviewed-on: http://gerrit.openafs.org/2880
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>