Workaround for fs_size on ZFS (ticket #46). ZFS reports incorrect st_blocks until file settles on disk, and this may take a while (i.e. just after creation of a file the st_blocks value is incorrect). As a workaround we now use st_blocks only if st_blocks * 512 > st_size, this should fix ZFS problems while still preserving accuracy for other filesystems. The problem had appeared in r3900 (1.0.1).