nanobsd: Account for metadata overhead in code slice

[jlduran]

Summary

The code slice size (CODE_SIZE) includes 16 sectors reserved for disk
metadata (see bsdlabel(8) offset). Subtract these 16 sectors from the
total size passed to nano_makefs.

This prevents the generated filesystem from consuming the entire slice
allocation, ensuring there is enough space for the metadata without
overflowing the partition boundary.

[imp]

ok, but maybe 16 is better encoded as a variable name.

[jlduran]

In D57220#1311210, @imp wrote:

ok, but maybe 16 is better encoded as a variable name.

Yes, will do!