btrfs: record delta directly in transaction_kthread

Rename 'now' to 'delta' and store there the delta between transaction
start time and current time. This is in preparation for optimising the
sleep logic in the next patch. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>