sched: make task_struct->plug always defined

If CONFIG_BLOCK isn't set, then it's an empty struct anyway. Just make
it generally available, so we don't break the compile:

kernel/sched/core.c: In function ‘sched_submit_work’:
kernel/sched/core.c:6346:35: error: ‘struct task_struct’ has no member named ‘plug’
 6346 |                 blk_flush_plug(tsk->plug, true);
      |                                   ^~
kernel/sched/core.c: In function ‘io_schedule_prepare’:
kernel/sched/core.c:8357:20: error: ‘struct task_struct’ has no member named ‘plug’
 8357 |         if (current->plug)
      |                    ^~
kernel/sched/core.c:8358:39: error: ‘struct task_struct’ has no member named ‘plug’
 8358 |                 blk_flush_plug(current->plug, true);
      |                                       ^~

Reported-by: Nathan Chancellor <nathan@kernel.org>
Fixes: 008f75a2 ("block: cleanup the flush plug helpers")
Signed-off-by: Jens Axboe <axboe@kernel.dk>