sched_rt: implement the .free_pdata hook

sched_rt: implement the .free_pdata hook

which is called by cpu_schedule_down(), and is necessary
for resetting the spinlock pointers in schedule_data from
the RTDS global runqueue lock, back to the default _lock
fields in the struct.

Not doing so causes Xen to explode, e.g., when removing
pCPUs from an RTDS cpupool and assigning them to another
one.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Meng Xu <mengxu@cis.upenn.edu>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>