sched/fair: Use dst_cpu's capacity rather than group {min, max} capacity

Comparing capacity extrema of local and source sched_group's doesn't make
much sense when at the day of the day the imbalance will be pulled by a
known env->dst_cpu, whose capacity can be anywhere within the local group's
capacity extrema.

Replace group_smaller_{min, max}_cpu_capacity() with comparisons of the
source group's min/max capacity and the destination CPU's capacity.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
[ionela.voinescu@arm.com: fix conflicts]