blk-iocost: Fix an UBSAN shift-out-of-bounds warning

When iocg_kick_delay() is called from a CPU different than the one which set
the delay, @now may be in the past of @iocg->delay_at leading to the
following warning:

UBSAN: shift-out-of-bounds in block/blk-iocost.c:1359:23
shift exponent 18446744073709 is too large for 64-bit type 'u64' (aka 'unsigned long long')
...
Call Trace:
<TASK>
dump_stack_lvl+0x79/0xc0
__ubsan_handle_shift_out_of_bounds+0x2ab/0x300
iocg_kick_delay+0x222/0x230
ioc_rqos_merge+0x1d7/0x2c0
__rq_qos_merge+0x2c/0x80
bio_attempt_back_merge+0x83/0x190
blk_attempt_plug_merge+0x101/0x150
blk_mq_submit_bio+0x2b1/0x720
submit_bio_noacct_nocheck+0x320/0x3e0
__swap_writepage+0x2ab/0x9d0

The underflow itself doesn't really affect the behavior in any meaningful
way; however, the past timestamp may exaggerate the delay amount calculated
later in the code, which shouldn't be a material problem given the nature of
the delay mechanism.

If @now is in the past, this CPU is racing another CPU which recently set up
the delay and there's nothing this CPU can contribute w.r.t. the delay.
Let's bail early from iocg_kick_delay() in such cases.

Reported-by: Breno Leitão <leitao@debian.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 5160a5a53c0c ("blk-iocost: implement delay adjustment hysteresis")
Link: https://lore.kernel.org/r/ZVvc9L_CYk5LO1fT@slm.duckdns.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>