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>