rx: Don't use app-thread variable in SendXmitList

rx: Don't use app-thread variable in SendXmitList

The value of call->app.mode is changed by the application thread
without taking the call lock. Instead of using this variable in
SendXmitList to determine whether the queue should be flushed, add
a new flag (RX_CALL_FLUSH) to control flushing behaviour.

As call->flags is manipulated under the call lock, its value can
be safely used by SendXmitList.

Change-Id: I8416697f457e5003af08a35ac08809512b03a9be
Reviewed-on: http://gerrit.openafs.org/8286
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>