a bug on: v4.20.1 : check/main.c:4728: add_data_backref: BUG_ON `!back` triggered, value 1 items checked)

[Leszek Dubiel <leszek.dubiel@xxxxxxxxxxxxxxx>]

As suggested on irc channel:

http://logs.tvrrug.org.uk/logs/%23btrfs/2019-03-11.html#2019-03-11T23:28:40

I should file a bug on btrfs tools:

check/main.c:4728: add_data_backref: BUG_ON `!back` triggered, value 1 
items checked)

after doing

btrfs check

on a btrfs file system that shows 355 GB (giga) free space, but when 
trying to do anything on files dumps error "no space left on device".