Module unloading issue - RTAI Master

[andypugh]

Reported in #3873

Just tested on latest master, RTAI builds and runs in my VM.

• latency-test works fine
• linuxcnc has some issues unloading the modules but works well otherwise with axis_mm. I think the order of unloading is wrong:

rmmod: ERROR: Module tpmod is in use by: motmod
<commandline>:0: exit value: 1
<commandline>:0: rmmod failed, returned -1
rmmod: ERROR: Module homemod is in use by: motmod
<commandline>:0: exit value: 1
<commandline>:0: rmmod failed, returned -1
rmmod: ERROR: Module trivkins is in use by: motmod
<commandline>:0: exit value: 1
<commandline>:0: rmmod failed, returned -1
<commandline>:0: unloadrt failed
rmmod: ERROR: Module hal_lib is in use by: trivkins homemod tpmod
rmmod: ERROR: Module rtapi is in use by: hal_lib trivkins homemod tpmod
rmmod: ERROR: Module rtai_math is in use by: homemod tpmod
rmmod: ERROR: Module rtai_sched is in use by: rtapi
rmmod: ERROR: Module rtai_hal is in use by: rtapi rtai_sched