Clean up orphaned indirdep dependency structures after disk failure.

During forcible unmount after a disk failure there is a bug that
causes one or more indirdep dependency structures to fail to be
deallocated. Until we manage to track down why they fail to get
cleaned up, this code tracks them down and eliminates them so that
the unmount can succeed.

Reported by: Peter Holm
Help from: kib
Reviewed by: Chuck Silvers
Tested by: Peter Holm
MFC after: 7 days
Sponsored by: Netflix