sched_rt: avoid ASSERT()ing on runq dump if there are no domains

sched_rt: avoid ASSERT()ing on runq dump if there are no domains

being serviced by the RTDS scheduler, as that is a
legit situation to be in: think, for instance, of a
newly created RTDS cpupool, with no domains migrated
to it yet.

While there:
 - move the spinlock acquisition up, to effectively
   protect the domain list and avoid races;
 - the mask of online pCPUs was being retrieved
   but then not used anywhere in the function: get
   rid of that.

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