Reduce the probability of low block numbers passed to ffs_snapblkfree() by skipping the call from ffs_snapremove() if the block number is zero.
Reduce the probability of low block numbers passed to ffs_snapblkfree() by
skipping the call from ffs_snapremove() if the block number is zero.

Simplify snapshot locking in ffs_copyonwrite() and ffs_snapblkfree() by using
the same locking protocol for low block numbers as for larger block numbers.
This removes a lock leak that could happen if vn_lock() succeeded after
lockmgr() failed in ffs_snapblkfree().

Check if snapshot is gone before retrying a lock in ffs_copyonwrite().

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