panic: softdep_count_dependencies during parallel installworld

While I was doing a parallel make installworld of a 15.1 build my
server crashed with

panic: softdep_count_dependencies: Unexpected type jnewblk

and the root filesystem was trashed.  I ended up recreating it so
there's no further debugging to be done on the disk image.  I saw a
lot of empty files and empty symlinks after running fsck -y to get
something mountable.  Bits of my source tree ended up in lost+found.

The stack trace of the panic thread was

softdep_count_dependencies
buf_countdeps
buf_flush
buf_daemon

Several other threads were working on the filesystem when they
were interrupted by an NMI telling them to stop.

Any thoughts?

John Carr