dashboard/configs/linux: enable BTRFS_DEBUG config

[tarasmadan]

Is your feature request related to a problem? Please describe.
BTRFS_DEBUG looks interesting.
Especially after the "btrfs: move ref-verify under CONFIG_BTRFS_DEBUG" change.
Some preliminary evaluation is needed to enable it.

[tarasmadan]

Regression in fs/btrfs/ref-verify.c (0 instrumentation) after #6382 could be recovered solving this issue.