I've experienced this exact same error at relocation.c:1449 running "4.12.14-lp150.12.58-default #1 openSUSE Leap 15.0". The call to btrfs_insert_root() within create_reloc_root() returns an error code which causes the subsequent BUG_ON() assertion to fail.

I'm hopeful that the following commit in the kernel will fix the underlying issue.

https://bugzilla.kernel.org/show_bug.cgi?id=203405
https://lkml.org/lkml/2019/6/7/720
https://github.com/torvalds/linux/co...a9570b6e23eb2b