ffs_blkfree: freeing free block -- ps, traces, fsck log

[Kirk McKusick]

My last checkin was in mid-March, so I am highly suspicious that
someone else has done something to create this problem. Unfortunately,
this panic comes about because of something that caused the bitmaps
to get trashed sometime (possibly much) earlier. The usual suspects
are updates to the bitmap that are failing (e.g., the I/O is not
being done), or in-memory trashing (e.g., wild pointers). Given the
nature of the fsck output, it looks like bitmap updates that are not
being properly done. A good quick test is to mount a filesystem, add
some files to it, unmount it and run fsck to see if any discrepencies
show up. As the unmount sometimes forces writes that would otherwise
fail, running a background fsck on it (fsck_ffs -B /foo) will 
sometimes show the problem a bit better.

	Kirk McKusick