sched_ext: Fix scx_bpf_dsq_peek() with FIFO DSQs

sched_ext: Fix scx_bpf_dsq_peek() with FIFO DSQs

When removing a task from a FIFO DSQ, we must delete it from the list
before updating dsq->first_task, otherwise the following lookup will
just re-read the same task, leaving first_task pointing to removed
entry.

This issue only affects DSQs operating in FIFO mode, as priority DSQs
correctly update the rbtree before re-evaluating the new first task.

Remove the item from the list before refreshing the first task to
guarantee the correct behavior in FIFO DSQs.

Fixes: 44f5c8ec5b9ad ("sched_ext: Add lockless peek operation for DSQs")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>