MFC of 269674:

The journal is only prepared to handle full-size block numbers, so we
have to adjust freeblk records to reflect the change to a full-size block.
For example, suppose we have a block made up of fragments 8-15 and
want to free its last two fragments. We are given a request that says:
    FREEBLK ino=5, blkno=14, lbn=0, frags=2, oldfrags=0
where frags are the number of frags to free and oldfrags are the number
of fragments to keep. To block align it, we have to change it to have a
valid full-size blkno, so it becomes:
    FREEBLK ino=5, blkno=8, lbn=0, frags=2, oldfrags=6

Submitted by: Mikihito Takehara
Tested by:    Mikihito Takehara
Reviewed by:  Jeff Roberson