mm: thp: simplify split_huge_pages_pid()

The helper find_get_task_by_vpid() can totally replace the task_struct
find logic in split_huge_pages_pid(), so use it to simplify the code. 
Also delete the needless comments for the helper function name already
explains what it's doing here.

Link: https://lkml.kernel.org/r/20240905153028.1205128-1-sunnanyong@huawei.comSigned-off-by: Nanyong Sun <sunnanyong@huawei.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/huge_memory.c

@@ -3819,16 +3819,11 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
 	vaddr_start &= PAGE_MASK;
 	vaddr_end &= PAGE_MASK;
 
-	/* Find the task_struct from pid */
-	rcu_read_lock();
-	task = find_task_by_vpid(pid);
+	task = find_get_task_by_vpid(pid);
 	if (!task) {
-		rcu_read_unlock();
 		ret = -ESRCH;
 		goto out;
 	}
-	get_task_struct(task);
-	rcu_read_unlock();
 
 	/* Find the mm_struct */
 	mm = get_task_mm(task);