sched/fair: Attempt misfit active balance when migration_type != migrate_misfit

Consider a 4-CPU big.LITTLE system with CPUs 0-1 as LITTLEs and CPUs 2-3 as
bigs. The resulting sched_domain hierarchy is:

  DIE [          ]
  MC  [    ][    ]
       0  1  2  3

When running a multithreaded CPU-bound workload (i.e. 1 hog per CPU), the
expected behaviour is to have the about-to-idle big CPUs pull a hog from
the LITTLEs, since bigs will complete their work sooner than LITTLEs.

Further Consider a scenario where:
- CPU0 is idle (e.g. its hog got migrated to one of the big CPUs)
- CPU1 is currently executing a per-CPU kworker, preempting the CPU hog
- CPU2 and CPU3 are executing CPU-hogs

CPU0 goes through load_balance() at MC level, and tries to pick stuff from
CPU1, but:
- the hog can't be pulled, because it's task_hot()
- the kworker can't be pulled, because it's pinned to CPU1, which sets
  LBF_SOME_PINNED

This load balance attempts ends with no load pulled, LBF_SOME_PINNED set,
and as a consequence we set the imbalance flag of DIE's [0, 1]
sched_group_capacity.

Shortly after, CPU2 completes its work and is about to go idle. It goes
through the newidle_balance(), and we would really like it to active
balance the hog running on CPU1 (which is a misfit task). However,
sgc->imbalance is set for the LITTLE group at DIE level, so the group gets
classified as group_imbalanced rather than group_misfit_task.

Unlike group_misfit_task (via migrate_misfit), the active balance logic
doesn't have any specific case for group_imbalanced, so CPU2 ends up going
idle. We'll have to wait for a load balance on CPU0 or CPU1 to happen and
clear the imbalance flag, and then for another DIE-level load-balance on
CPU2 to happen to pull the task off of CPU1. That's several precious
milliseconds wasted down the drain.

Giving group_misfit_task a higher group_classify() priority than
group_imbalance doesn't seem like the right thing to do. Instead, make
need_active_balance() return true for any migration_type when the
destination CPU is idle and the source CPU has a misfit task.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>