Tree as of commit 7bc4a4ce68f8c6d064ea949446852e996526f692 dies during 
btrfs build on x86-64:

   CC [M]  fs/btrfs/ctree.o
In file included from fs/btrfs/ctree.c:21:0:
fs/btrfs/ctree.h:1003:17: error: field ‘super_kobj’ has incomplete type
fs/btrfs/ctree.h:1074:17: error: field ‘root_kobj’ has incomplete type
make[2]: *** [fs/btrfs/ctree.o] Error 1
make[1]: *** [fs/btrfs] Error 2
make: *** [fs] Error 2

Config attached.