sched_ext: Add SCX_TASK_REENQ_REASON flags

sched_ext: Add SCX_TASK_REENQ_REASON flags

SCX_ENQ_REENQ indicates that a task is being re-enqueued but doesn't tell the
BPF scheduler why. Add SCX_TASK_REENQ_REASON flags using bits 12-13 of
p->scx.flags to communicate the reason during ops.enqueue():

- NONE: Not being reenqueued
- KFUNC: Reenqueued by scx_bpf_dsq_reenq() and friends

More reasons will be added.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>