Fix bug 253158 - Panic: snapacct_ufs2: bad block - mksnap_ffs(8) crash

The panic reported in 253158 arises because the /mnt/.snap/.factory
snapshot allocated the last block in the filesystem. The snapshot
code allocates the last block in the filesystem as a way of setting
its length to be the size of the filesystem. Part of taking a
snapshot is to remove all the earlier snapshots from the image of
the newest snapshot so that newer snapshots will not claim the blocks
of the earlier snapshots. The panic occurs when the new snapshot
finds that both it and an earlier snapshot claim the same block.

The fix is to set the size of the snapshot to be one block after
the last block in the filesystem. This block can never be allocated
since it is not a valid block in the filesystem. This extra block
is used as a place to store the initial list of blocks that the
snapshot has already copied and is used to avoid a deadlock in and
speed up the ffs_copyonwrite() function.

Reported by:  Harald Schmalzbauer
Tested by:    Peter Holm
PR:           253158
Sponsored by: Netflix

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