sched/topology: Skip duplicate group rewrites in build_sched_groups()

While staring at build_sched_domains(), I realized that get_group()
does several duplicate (thus useless) writes.

If you take the Arm Juno r0 (LITTLEs = [0, 3, 4, 5], bigs = [1, 2]), the
sched_group build flow would look like this:

('MC[cpu]->sg' means 'per_cpu_ptr(&tl->data->sg, cpu)' with 'tl == MC')

build_sched_groups(MC[CPU0]->sd, CPU0)
  get_group(0) -> MC[CPU0]->sg
  get_group(3) -> MC[CPU3]->sg
  get_group(4) -> MC[CPU4]->sg
  get_group(5) -> MC[CPU5]->sg

build_sched_groups(DIE[CPU0]->sd, CPU0)
  get_group(0) -> DIE[CPU0]->sg
  get_group(1) -> DIE[CPU1]->sg <=================+
						  |
build_sched_groups(MC[CPU1]->sd, CPU1)            |
  get_group(1) -> MC[CPU1]->sg                    |
  get_group(2) -> MC[CPU2]->sg                    |
						  |
build_sched_groups(DIE[CPU1]->sd, CPU1)           ^
  get_group(1) -> DIE[CPU1]->sg  } We've set up these two up here!
  get_group(3) -> DIE[CPU0]->sg  }

From this point on, we will only use sched_groups that have been
previously visited & initialized. The only new operation will
be which group pointer we affect to sd->groups.

On the Juno r0 we get 32 get_group() calls, every single one of them
writing to a sched_group->cpumask. However, all of the data structures
we need are set up after 8 visits (see above).

Return early from get_group() if we've already visited (and thus
initialized) the sched_group we're looking at. Overlapping domains
are not affected as they do not use build_sched_groups().

Tested on a Juno and a 2 * (Xeon E5-2690) system.

( FWIW I initially checked the refs for both sg && sg->sgc, but figured if
  they weren't both 0 or > 1 then something must have gone wrong, so I
  threw in a WARN_ON(). )

No change in functionality intended.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>