sched: small optimisation of can_migrate_task()

There were 3 invocations of task_hot() in can_migrate_task().

Replace these 3 invocations by only one invocation, cached in
a local variable.

Signed-off-by: Luis Henriques <henrix@sapo.pt>
LKML-Reference: <20090316195902.GA6197@hades.domain.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>