Wow. I've never seen such a straight-forward post in a Slackware forum go unanswered for so long.
Now that I know better, I'll never post in the "Slackware - Installation" forum again.
That's for sure.
A simple,quick search for btrfs yielded 0 results at the main page of docs.slackware.com too.
Here's the solution:
When presented with the prompt in the pic, enter the root password, then:
# mount -n -o remount,rw /
# cp /bin/true /sbin/btrfs.fsck
Then edit /etc/fstab and change fs_passno to "0" for all mount points that are btrfs (That's the sixth field).