sched/fair: Fix capacity and nr_run comparisons in can_migrate_task()

Kernel version 3.18 and beyond alter the definition of sgs->group_capacity
whereby it reflects the load a group is capable of taking. In previous
kernel versions the term used to refer to the number of effective CPUs
available. This change breaks the comparison of capacity with the number
of running tasks on a group. To fix this convert the capacity metric
before doing the comparison.

Change-Id: I3ebd941273edbcc903a611d9c883773172e86c8e
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>