sched:/fair: check cpu_allowed mask when find target

Current find best target cpu is not check cpu_allowed mask, so
it is possible to place the task on wrong cpu, add cpu mask
check when find best target.

Change-Id: I8075657fa67704ab8d9d94d542a7983bcf8d034f
Signed-off-by: Biao Long <blong@codeaurora.org>