parametrized in the filesystem, defaulting to MIN_FREE_SEGS = 2 but set
to something more reasonable at newfs_lfs time.
Note the number of blocks that have been scheduled for writing but which
are not yet on disk in an inode extension, i_lfs_effnblks. Move
i_ffs_effnlink out of the ffs extension and onto the main inode, since
it's used all over the shared code and the lfs extension would clobber
it.
At inode write time, indirect blocks and inode-held blocks of inodes
that have i_lfs_effnblks != i_ffs_blocks are cleansed of UNWRITTEN disk
addresses, so that these never make it to disk.
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