mm/kmemleak: use PF_KTHREAD flag to detect kernel threads

mm/kmemleak: use PF_KTHREAD flag to detect kernel threads

Replace the current->mm check with PF_KTHREAD flag for more reliable
kernel thread detection in scan_should_stop().  The PF_KTHREAD flag is the
standard way to identify kernel threads and is not affected by temporary
mm borrowing via kthread_use_mm() (although kmemleak does not currently
encounter such cases, this makes the code more robust).

No functional change.

Link: https://lkml.kernel.org/r/20260130093729.2045858-3-zhongqiu.han@oss.qualcomm.com
Signed-off-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>