Backport - sched: always use blk_schedule_flush_plug in io_schedule_out

sched: always use blk_schedule_flush_plug in io_schedule_out
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/kernel/sched/core.c?id=10d784e

I drive it with 4.0.4-505.fc21.x86_64, no problemos. Please add it in the stable tree, 4.0.x.

Ref.
https://bugzilla.kernel.org/show_bug.cgi?id=98191