tools/sched_ext: Fix off-by-one in scx_sdt payload zeroing

scx_alloc_free_idx() zeroes the payload of a freed arena allocation
one word at a time. The loop bound was alloc->pool.elem_size / 8, but
elem_size includes sizeof(struct sdt_data) (the 8-byte union sdt_id
header). This caused the loop to write one extra u64 past the
allocation, corrupting the tid field of the adjacent pool element.

Fix the loop bound to (elem_size - sizeof(struct sdt_data)) / 8 so
only the payload portion is zeroed.

Test plan:
- Add a temporary sanity check in scx_task_free() before the free call:

if (mval->data->tid.idx != mval->tid.idx)
scx_bpf_error("tid corruption: arena=%d storage=%d",
mval->data->tid.idx, (int)mval->tid.idx);

- stress-ng --fork 100 -t 10 & sudo ./build/bin/scx_sdt

Without this fix, running scx_sdt under fork-heavy load triggers the
corruption error. With the fix applied, the same workload completes
without error.

Fixes: 36929ebd17ae ("tools/sched_ext: add arena based scheduler")
Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>