btrfs: discard: store async discard delay as ns not as jiffies

Most delay calculations are done in ns or ms, so store
discard_ctl->delay in ms and convert the final delay to jiffies only at
the end.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>