stress2: add a reproducer for the flush_newblk_dep() "Bad newblk" panicHEADmain

flush_pagedep_deps() drops the soft updates lock to obtain the vnode of
a new directory.  If its MKDIR_BODY dependency completes in that window,
the lookup of the directory's first block in flush_newblk_dep() can find
a stale allocindir left behind by a previous owner of the same block,
relocated by ffs_reallocblks() and freed, and panic.

The test grows interleaved files past UFS_NDADDR to keep clusters being
relocated, while other workers create subdirectories in a parent with
IN_ENDOFF set, so that ffs_vput_pair() syncs it, and fsync() them to
complete the mkdir dependencies.  The file system layout and the way
the writers put their blocks on disk are arranged so that a new
directory takes over a freed block whose dependency is still retained;
the details are in the script.  With dtrace=1, the test also counts how
often this precondition is met, which works on a fixed kernel too.

PR:		297976
Reviewed by:	kib, pho
Tested by:	pho
Sponsored by:	Sippy Software, Inc.
Differential revision:	https://reviews.freebsd.org/D59356

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