I've had failures with several kernel compiling attempts using kernel-kit master, over the last few weeks.
The kernel compiles, and when booted is ok. When you exit, crashes, with error: au_fsctx_parse_monolithic
I've just discovered there is a bug, which @peebee and sfjro just tested.
....aufs_sources/fs/aufs/fsctx.c has an error (in line 1094?): while (1) should be while (str)
See: https://github.com/sfjro/aufs5-standalone/issues/9
Recent 5.10, 5.15 & 5.16 kernels affected. 2 patch tries were posted on github.
EDIT: 14/3/22 - BUG NOW FIXED - PATCHES NO LONGER NEEDED.