ffs_blkfree bug

[Nathan Whitehorn]

This problem triggers data corruption, as well as panics: it just 
destroyed nearly all the partitions on a machine of mine (hooray for backups).
It was running 7-CURRENT from a week before the branch on amd64 with an 
LSI SAS controller.

The interesting thing is the nearly all -- it destroyed all the partitions 
with softupdates on, and left all the ones with softupdates off unharmed. 
Maybe this is useful?
-Nathan

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them)