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

makefs(1) has a number of signedness warnings (when built with higher
WARNS), most of which can be addressed by careful application of casts
in makefs itself.

There is one case where a signedness warning arises from the blksize
macro, so must be addressed in the macro itself.

Reviewed by: kib, mckusick
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10589