block: limit block time caching to in_task() context

We should not have any callers of this from non-task context, but Jakub
ran [1] into one from blk-iocost. Rather than risk running into others,
or future ones, just limit blk_time_get_ns() to when it is called from
a task. Any other usage is invalid.

[1] https://lore.kernel.org/lkml/CAHk-=wiOaBLqarS2uFhM1YdwOvCX4CZaWkeyNDY1zONpbYw2ig@mail.gmail.com/

Fixes: da4c8c3d ("block: cache current nsec time in struct blk_plug")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk.h

@@ -534,7 +534,7 @@ static inline u64 blk_time_get_ns(void)
 {
 	struct blk_plug *plug = current->plug;
 
-	if (!plug)
+	if (!plug || !in_task())
 		return ktime_get_ns();
 
 	/*