rx: Lock call for KeepAliveOn/KeepAliveOff

rx: Lock call for KeepAliveOn/KeepAliveOff

rxi_KeepAliveOn/Off expect the call lock to be held after the call has
been initialized. So, hold it in the rx_KeepAliveOn/Off callers.

Change-Id: Ic2e96853bc9553e97f62e4502a5c92bbb28c63a2
Reviewed-on: http://gerrit.openafs.org/8463
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>