UFS fs.h: clear warning from use by makefs(1)

[emaste]

Summary

A different approach to cleaning up makefs warnings will change types or casts there, as opposed to the change in D10578. There is one case where a signedness warning arises from a macro in fs.h, so must be addressed in the macro itself.

[kib]

I checked all users of blksize().

[mckusick]

I like this solution much better than D10578.