Btrfs with the ref verify tool compiled in

configname: CONFIG_BTRFS_FS_REF_VERIFY

Linux Kernel Configuration
└─>File systems
└─>Btrfs with the ref verify tool compiled in
In linux kernel since version 4.2 (release Date: 2015-08-30)  
Enable run-time extent reference verification instrumentation. This
is meant to be used by btrfs developers for tracking down extent
reference problems or verifying they didn't break something.

If unsure, say N.