bcache: writeback rate clamping: make 32 bit safe

Sorry this got through to linux-block, was detected by the kbuilds test
robot.  NSEC_PER_SEC is a long constant; 2.5 * 10^9 doesn't fit in a
signed long constant.

Fixes: e41166c5 ("bcache: writeback rate shouldn't artifically clamp")
Reviewed-by: Coly Li <colyli@suse.de>
Signed-off-by: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>